Geochemistry and geodynamic origin of the Mesoarchean Ujarassuit

Lithos 113 (2009) 133–157

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Geochemistry and geodynamic origin of the Mesoarchean Ujarassuit and Ivisaartoqgreenstone belts, SW Greenland

J.C. Ordóñez-Calderón a,⁎, A. Polat a, B.J. Fryer a,b, P.W.U. Appel c, J.A.M. van Gool c, Y. Dilek d, J.E. Gagnon a,b

a Department of Earth and Environmental Sciences, University of Windsor, Windsor, ON, Canada N9B 3P4b Great Lakes Institute for Environmental Research, University of Windsor, Windsor, ON, Canada N9B 3P4c Geological Survey of Denmark and Greenland, 1350 Copenhagen, Denmarkd Department of Geology, Miami University, Oxford, OH 45056, USA

⁎ Corresponding author. Tel.: +1 519 253 3000x2486E-mail address: [email protected] (J.C. Ordóñez

0024-4937/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.lithos.2008.11.005

a b s t r a c t
a r t i c l e i n f o
Article history:
The Ivisaartoq (ca. 3075 M Received 23 May 2008Accepted 5 November 2008Available online 21 November 2008
Keywords:Mesoarchean oceanic crustGreenstone beltSupra-subduction zoneAmphiboliteBoninitePicrite

a) and Ujarassuit (ca. 3070 Ma) greenstone belts are the largest Mesoarcheansupracrustal lithotectonic assemblages in the Nuuk region, SW Greenland. Both greenstone belts underwentpolyphase deformation and amphibolite facies metamorphism, and were in due course variouslydismembered. Pillow lavas, pillow breccia, magmatic layering, and relic sedimentary structures are wellpreserved in the Ivisaartoq belt. Volcanic rocks include basalts, and minor andesites and picrites. Boninite-like rocks occur in the Ujarassuit belt. Metasedimentary rocks in the Ujarassuit belt occur as thin layers (0.5–1.0 m) of biotite schists, and in the Ivisaartoq belt as ~500 m-thick biotite schists intercalated with minorquartzitic gneisses. There is no field evidence indicating that the Ivisaartoq and Ujarassuit supracrustal rockswere deposited on older continental basement, and their volcanic rocks do not exhibit any geochemicaltrends indicating contamination by Archean upper continental crust.Four groups of amphibolites (metavolcanic rocks) were recognized in the Ujarassuit greenstone belt onthe basis of their REE and HFSE characteristics: (1) Group 1 is characterized by near-flat REE patterns(La/Smcn=0.77–1.14; La/Ybcn=0.84–1.24) and moderate negative Nb anomalies (Nb/Nb⁎=0.60–0.79); (2)Group 2 displays LREE-depleted patterns (La/Smcn=0.53–1.02; La/Ybcn=0.32–0.61) and pronounced negativeNb anomalies (Nb/Nb⁎= 0.32–0.67); (3) Group 3 consists of LREE depleted patterns (La/Smcn=0.69–0.84;La/Ybcn=0.55–0.91) and absence of significant Nb anomalies (Nb/Nb⁎=0.92–1.15); and (4) Group 4 hasconcave-upward REE patterns (La/Smcn=1.64–2.42, Gd/Ybcn=0.57–1.01) and large negative Nb anomalies(Nb/Nb⁎=0.28–0.42). The depletion of LREE in Group 2 amphibolites appears to have resulted frommobility ofthese elements during metamorphism. Group 1, 3, and 4 amphibolites retain their near-primary geochemicalsignatures and their trace element patterns are comparable to those of Phanerozoic oceanic island arctholeiites (IAT), normal-mid-ocean ridge basalts (N-MORB), and boninites, respectively. The trace elementpatterns of the least alteredmeta-ultramafic rocks (La/Ybcn=2.48–1.35; Nb/Nb⁎=0.31–0.60) are comparable tothose of modern subduction-related picrites. Amphibolites with an andesitic composition show large negativeNb anomalies (Nb/Nb⁎=0.21–0.55), enriched LREE patterns (La/Ybcn=6.29–15.64), and fractionated HREE(Gd/Ybcn=2.61–3.12) implying deep melting in equilibrium with residual garnet in the source. Biotite schistsand quartzitic gneisses have low chemical indexes of alteration values (CIA=46 to 62) and trace elementcharacteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to maficsource rocks. Collectively, the geochemical features of metavolcanic and metasedimentary rocks suggest thatthe Ivisaartoq and Ujarassuit greenstone belts represent dismembered fragments of Mesoarchean supra-subduction zone oceanic crust formed in an arc-forearc-backarc tectonic setting.

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1. Introduction

Archean greenstone belts are composed dominantly of volcanicrocks and include a significant fraction of siliciclastic, volcaniclastic, andchemical sedimentary rocks (Condie, 1994). Basaltic lava flows areprevalent and are commonly associated with felsic, intermediate, and

.-Calderón).

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ultramafic volcanic rocks. The lithogeochemical diversity of Archeangreenstone belts is the product of complex igneous and sedimentolo-gical processes operating in a wide variety of tectonic settings either atintra-oceanic or intra-continental environments (Eriksson and Catu-neanu, 2004). Accordingly, it has been suggested that greenstone beltsmay represent the volcanic remnants of Archean intra-oceanic islandarcs, active continental margins, mid-ocean ridges, large igneousprovinces, and intra-continental rifts (Bickle et al., 1994; Ohta et al.,1996; Polat et al., 1998; Polat and Kerrich, 2001; Hartlaub et al., 2004;

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Fig. 1. Geological map of the Ivisaartoq and Ujarassuit greenstone belts with approximate sample locations (modified from Friend and Nutman, 2005 after Chadwick and Coe, 1988).The location of field photographs are also indicated (Figs. 2a–h and 3a–h). Structural observations and sampling of metavolcanic and metasedimentary rocks were conducted in theeastern and western flanks of the Ujarassuit greenstone belt. The lower metasedimentary unit of the Ivisaartoq greenstone belt was also sampled.

134 J.C. Ordóñez-Calderón et al. / Lithos 113 (2009) 133–157

Sandeman et al., 2004; Thurston and Ayres, 2004; Kusky, 2004). Therecognition of intra-oceanic Archean greenstone belts is of funda-mental importance because they provide invaluable information on the

Table 1Mineralogical characteristics and interpreted protoliths for the main rock types discussed in

Lithology Mineralogical composition

Hornblende-rich amphibolites(Groups 1–3 amphibolites)

Hornblende+plagioclase+quartz+zircon+apatite±magnetite±garnet±biotite±cum

Cummingtonite-rich amphibolites(Group 4 amphibolites)

Cummingtonite±anthophyllite±hornblendplagioclase+quartz±biotite±magnetite

Plagioclase-rich amphibolites Hornblende+plagioclase+quartz+zircon+apatite±magnetite±biotite

Serpentinites Serpentine+talc+actinolite+tremolite+mcummingtonite±olivine ±clinopyroxene

Actinolite–tremolite-rich amphibolites Actinolite+tremolite+magnetite±cumminhornblende±clinopyroxene±magnetite

Biotite schists Biotite+plagioclase+quartz+magnetite±gepidote±titanite±muscovite±microcline±cummingtonite±anthophyllite±tourmaline

Quartzitic gneisses Quartz+plagioclase+biotite +zircon+muhornblende±magnetite±tourmaline

geodynamic origin of the Archean oceanic crust. In addition, given thatoceanic volcanic rocks are less susceptible to crustal contaminationthan greenstone belts deposited on continental crust, they also provide

this study.

Protolith

titanite+mingtonite

Basaltic rocks. Groups 1–2 amphibolites represent islandarc tholeiites (IAT). Group 3 amphibolites represent rareN-MORB-like tholeiites

e+ Boninite-like rocks

titanite+ Transitional to calc-alkaline basaltic andesites and andesites

agnetite± Olivine-rich pricritic cumulus

gtonite± Picritic volcanic rocks

arnet±hornblende±

Immature volcanoclastic graywackes derived from maficto felsic source rocks

scovite±garnet± Quartz-rich arkoses derived from felsic source rocks

135J.C. Ordóñez-Calderón et al. / Lithos 113 (2009) 133–157

information on thermal and geochemical characteristics of the Archeanmantle (Bennett et al., 1993; Ohta et al., 1996; Pollack, 1997; Kerrichet al., 1999; Polat et al., 1999; Komiya et al., 2004; Condie, 2005a).

Fig. 2. Field photographs of various rock types in the Ivisaartoq (a–c) and Ujarassuit (d–h)in biotite schist of the Ivisaartoq greenstone belt. (d) Serpentinized olivine-richmeta-ultrama(f–g) Migmatitic TTG–gneiss, close to the contact with supracrustal rocks, with large hornblerecumbent F3 folding.

However, the intra-oceanic origin of Archean greenstone belts iscontroversial because their original stratigraphic relationships andprimary geochemical signatures have been variably modified during

greenstone belts. (a) Pillow basalt from the Ivisaartoq belt. (b–c) Relict felsic cobblesfic rockwith lens-shaped geometry. (e) Biotite schist with small-scale F3 isoclinal folds.nde-rich amphibolite xenolith. (h) Migmatitic hornblende-rich amphibolite affected by


hydrothermal alteration, regional metamorphism, polyphase deforma-tion, and plutonism (Fryer et al., 1979; Gruau et al., 1992; Lahaye et al.,1995; Wilkins, 1997; Polat et al., 2003; Weiershäuser and Spooner,2005).

The Nuuk region in southern West Greenland comprises severalearly to late Archean (3850–2800 Ma) tectono-stratigraphic terranesassembled into a single block in the late Archean (Friend et al., 1988,1996; Nutman et al., 1989; McGregor et al., 1991; Friend and Nutman,2005; Garde, 2007). The diachronous accretion of allochtonous terranesin the Nuuk region is one of the best documented examples of Archeancollisional orogeny (Nutman and Friend, 2007). The collisional tectonicmodel proposed for the Nuuk region suggests that some greenstonebelts may represent relict fragments of Archean ocean floor accretedonto continental crust during the closure of Archean ocean basins in thelate stages of a Wilson cycle (cf. Casey and Dewey, 1984; Şengör, 1990).High-grade middle- to lower-crustal rocks are exposed in the Nuukregion at present. Therefore, the volcanic stratigraphy of mostsupracrustal belts is incomplete given that the uppermost crustal levelshave been removed by erosion (Garde, 2007). Nevertheless, the regionincludes significant remnants of Mesoarchean intra-oceanic volcanicsuites represented by rare island arc complexes (ca. 3071Ma) in Qussukand Bjørneøen (Garde, 2007) and incomplete fragments of supra-subductionzoneoceanic crust (ca. 3075Ma) in the Ivisaartoqgreenstonebelt (Polat et al., 2007, 2008).

The geochemical characteristics of metavolcanic rocks in theIvisaartoq belt have been investigated by Polat et al. (2007, 2008)andOrdóñez-Calderón et al. (2008). However, nomodern geochemicalstudies have been conducted in supracrustal rocks of the UjarassuitNunaat area to the NNWof the Ivisaartoq belt (Fig. 1). These rocks areunofficially named as the Ujarassuit greenstone belt in this study. TheUjarassuit greenstone belt is dominated by hornblende-rich amphi-bolites with a basaltic composition (Fig. 1). The belt also includesvolumetrically minor basaltic andesites, andesites, picrites, boninites,and volcanoclastic sedimentary rocks which are now, respectively,plagioclase-rich amphibolites, serpentinites, actinolite–tremolite-richamphibolites, cummingtonite-rich amphibolites, biotite schists, andquartzitic gneisses (Table 1). This diverse lithological association pro-vides an excellent opportunity to investigate Archean volcanogenicprocesses and geodynamic depositional environments.

In this contribution, we report new high-precision major and traceelement data for 34 samples of metavolcanic rocks in the Ujarassuitgreenstone belt with the followingobjectives: (1) to assess the effects ofhigh-grademetamorphismandpostmagmatic alterationon the primarygeochemical signatures; (2) to investigate source characteristics andmantle processes; (3) to understand the geodynamic origin; and (4) forinter-comparisonwithwell studiedmetavolcanic rocks of the Ivisaartoqand Qussuk belts (Garde, 2007; Polat et al., 2007, 2008; Ordóñez-Calderón et al., 2008). In addition, new geochemical data have beenreported for 13 samples from metavolcanoclastic–sedimentary rocks

Table 2Sequence of ductile deformation events at upper amphibolite facies conditions in the Ujara

Deformation phase Structure Description

D4 F4 Late upright folds with ENE-trending axial plane (F

D3 S3 NNW- to NNE-trending, steeply-dipping, regional trocks. S3 is likely a composite foliation which resu

F3 Tight isoclinal and recumbent folds plunging 20°–7

D2 S2 Earliest tectonic foliation developed in TTG–gneisssupracrustal rocks was transposed into parallelism

F2 Now preserved as rare relict rootless folds. S1 is tig

D1 S1 Oldest tectonic foliation preserved in the belt. It isamphibolite xenoliths included within the TTG–gn

in the Ivisaartoq and Ujarassuit greenstone belts to understand theirprovenance and to provide additional constraints on the depositionalsetting of these supracrustal belts.

2. Geological setting and field characteristics

Structural and U–Pb zircon geochronological studies have shownthat the Nuuk region is composed of several Eo- to Neoarchean (3850–2800 Ma) tectono-stratigraphic terranes (Fig. 1) bounded by amphi-bolite facies mylonites (Friend et al., 1987, 1988; Nutman et al., 1989;McGregor et al., 1991; Friend et al., 1996; Crowley, 2002; Friend andNutman, 2005). These allochtonous terranes comprise associations oftonalite–trondhjemite–granodiorite (TTG) gneisses and fragments ofgreenstone belts (Black et al., 1971; Moorbath et al., 1973; McGregor,1973; Bridgwater et al., 1974; McGregor and Mason, 1977; Chadwick,1990; Nutman et al., 1996; Garde, 2007; Polat et al., 2007). In theMeso- and Neoarchean, the region underwent multiple phases ofdeformation and metamorphism at upper amphibolite facies condi-tions, which appears to have been related to several episodes ofterrane accretion operating between 2960 Ma and 2710 (Friend andNutman, 1991; Friend et al., 1996; Friend and Nutman, 2005).

The Ivisaartoq greenstone belt is located within the ca. 3070–2970 Ma Kapisilik terrane (Fig. 1) (Chadwick, 1985, 1990; Friend andNutman, 2005). A minimum depositional age of ca. 3075 Ma has beenconstrained by U–Pb zircon geochronology in felsic volcaniclastic–sedimentary rocks (Friend and Nutman, 2005; Polat et al., 2007). Thebelt comprises a sequence of metamorphosed pillow basalts, withminor intercalations of picrites, gabbros, and volcaniclastic–sedimentaryrocks (Hall, 1980; Chadwick, 1985, 1986 1990; Polat et al., 2007, 2008).The Ivisaartoq belt has been subdivided into two lithotectonic groups(Chadwick, 1990). The upper group is less deformed than the lowergroup and contains primary magmatic features including pillow lavas(Fig. 2a), cumulate textures, and volcanic breccia (Chadwick, 1990;Polat et al., 2007, 2008). These primary features are rare in the lowerlithotectonic group. The base of the lower group includes a ca. 500 mthick unit of biotite schists and quartzitic gneisses interpreted asvolcaniclastic–sedimentary rocks (Chadwick, 1990; this study) (Fig. 1).Biotite schists are the most abundant rock type (Table 1). Layers ofquartzitic gneisses up to 1m thick are locally intercalated.Despite strongdeformation and penetrative foliation, the biotite schists preserverounded felsic cobbles 5 to 20 cm in length (Fig. 2b–c). In addition, afew layers of hornblende-rich amphibolites (up to 50 cm thick) parallelto the prevalent foliation are intercalated within this volcaniclastic–sedimentary unit.

The Ujarassuit greenstone belt appears to be the northerncontinuation of the Ivisaartoq belt (Fig. 1) (Hall and Friend, 1979;Chadwick, 1990; Friend and Nutman, 2005). The belt is less than 1 kmwide owing to strong tectonic attenuation. Tonalite–trondhjemite–granodiorite plutons, now TTG–gneisses, intrude the Ujarassuit

ssuit greenstone belt.

ig. 3d). Amplitude of meters to several kilometers. Reorientation of D1–D3 structures.

ectonic foliation. This foliation is conformable to both TTG–gneisses and supracrustallted from high strain deformation and reorientation of previous planar structures.

0° SSE to SSW (Figs. 2h and 3b–c). Amplitude of centimeters to hundred meters.

es that intrude into the Ujarassuit greenstone belt. The S1 tectonic foliation inwith S2.

htly folded around this generation of folds (Fig. 3a). Amplitude of centimeters.

only developed in amphibolites. Preferentially preserved in hornblende-richeisses (Fig. 2g).


greenstone belt. These TTG–gneisses have yielded Mesoarchean U–Pbzircon ages between 3070 and 2972 Ma (Friend and Nutman, 2005;Hollis et al., 2006a). These ages have been interpreted as the age of the

Fig. 3. Field photographs of various rock types in the Ujarassuit greenstone belt. (a) HornblenS3 foliation. (b–c) F3 recumbent fold in hornblende-rich amphibolite. Layers of calc-silicate alhornblende-rich amphibolite. (e–f) Asymmetric and transposed folds in mylonites at the cpyrite-bearing quartz-rich layers hosted by hornblende-rich amphibolite.

magmatic event, and provide a minimum depositional age that iscomparable with the ages reported for the Ivisaartoq greenstone belt(Friend and Nutman, 2005; Polat et al., 2007).

de-rich amphibolite with rootless F1 isoclinal fold transposed into the prevalent regionalteration have been affected by F2 recumbent folding. (d) Outcrop-scale F4 upright fold inontact between TTG–gneiss and hornblende-rich amphibolite. (g–h) Calc-silicate and


The Ujarassuit greenstone belt is dominated by hornblende-richamphibolites (Table 1). They display homogenous or layered appearanceon outcrop scale owing to variations in the abundance of plagioclase andhornblende. Primary volcanic features are not preserved. Plagioclase-richamphibolites, serpentinites, and actinolite–tremolite-rich amphibolitesare locally intercalatedwith hornblende-rich amphibolites (Table 1). Theserpentinites and actinolite–tremolite-rich amphibolites (meta-ultrama-fic rocks) occur as discontinuous boudinaged lenses 1m to 1 km in length(Fig. 2d). They are strongly altered, and rarely contain fresh olivine andclinopyroxene. Metagabbros with relict igneous textures are locallypreserved. Biotite schists are minor components of the Ujarassuit green-stone belt (Fig. 2e). They occur as 0.5 to 1m thick layers intercalatedwithhornblende-rich amphibolites. In some areas, this intercalation formspackages up to 30 m thick. Biotite schists are petrographically similar

Fig. 4. Photomicrographs of metavolcanic and metasedimentary rocks (see Table 1). (aamphibolite (Group 4 amphibolites). (c) Serpentinized olivine-rich meta-ultramafic rock. (accessory garnet. (f) Fine-grained quartzitic gneiss with abundant muscovite and minor biotAbbreviations: Bt, biotite; Cum, cummingtonite; Grt, garnet; Hbl, hornblende; Ms, muscovi

to those exposed in the lower volcaniclastic–sedimentary unit of theIvisaartoq greenstone belt (Chadwick, 1990). Rare cummingtonite-rich amphibolites are intercalated with hornblende-rich amphibolitesand meta-ultramafic rocks. The surrounding TTG–gneisses are stronglymigmatitic (Fig. 2f). TTG–gneisses from the Kapisilik terrane intrude intothe supracrustal rocks and include amphibolite xenoliths (Fig. 2g) (Holliset al., 2006b; this study). Hornblende-rich amphibolites are also variablymigmatitic (Fig. 2h).

The Ujarassuit belt exhibits evidence for at least four major phasesof ductile deformation at upper amphibolite facies metamorphicconditions (Table 2) (cf. Hall and Friend, 1979; Friend and Nutman,1991; Hollis et al., 2006b). The oldest deformation event (D1) isindicated byan earlier tectonic foliation (S1) developed in supracrustalrocks, which is locally preserved in amphibolite xenoliths within TTG–

) Hornblende-rich amphibolite (Groups 1–3 amphibolites). (b) Cummingtonite-richd) Unaltered olivine-rich meta-ultramafic rock (see also Fig. 3e). (e) Biotite schist withite. Plane polarized light for (a–b) and (e), and crossed polarized light for (c–d) and (f).te; Pl, plagioclase; Qtz, quartz.


gneisses from the Kapisilik terrane (Fig. 2g). During a second phase ofdeformation (D2), the S1 foliation was folded around now relictrootless folds (F2) (Fig. 3a). The tectonic foliation (S2) of the TTG–gneisses that intrude the Ujarassuit belt was likely formed during D2deformation. A third phase of deformation (D3) resulted in tight F3isoclinal and recumbent folds (Fig. 3b–c). Felsic leucosome inmigmatitic amphibolites is folded around F3 recumbent folds(Fig. 2h). The regional tectonic foliation (S3) likely resulted from thisdeformation event. The latest phase of ductile deformation (D4) ischaracterized by upright F4 foldswhich refolded and reoriented earliertectonic structures (Fig. 3d). The orientations of D3 and D4 structuresin the supracrustal rocks and TTG–gneisses are conformable.

The contacts between amphibolites and TTG–gneisses are partiallydelimited by high grade mylonites (Fig. 3e–f) up to 0.5 m in thickness.Rare kinematic indicators are consistent with a dextral SSW tectonictransportation (Fig. 3e–f).

The Ujarassuit greenstone belt displays evidence for hydrothermalalteration, which is indicated by localized concordant calc-silicaterocks and layers of pyrite-bearing quartz-rich rocks (Fig. 3g–h). Thecalc-silicate rocks are parallel to the regional foliation, and have been

Table 3Measured and recommended major and trace element concentrations for USGS standards W

W-2

Recommended Measured (n=5) RSD

SiO2 (wt.%) 52.68 52.11 1.4TiO2 1.06 1.07 0.8Al2O3 15.45 15.23 0.9Fe2O3 10.83 10.74 0.8MnO 0.17 0.17 1.3MgO 6.37 6.34 0.3CaO 10.86 10.87 0.5Na2O 2.20 2.24 1.2K2O 0.63 0.62 1.9P2O5 0.14 0.15 9.8Total 99.89 99.54 0.9Sc (ppm) 36.0 35.3 2.7Zr 94.0 88.0 4.0

BHVO-1

Recommended Measured (n=12) RSD (

V (ppm) 298.2 22.4Cr 149.6 21.5Co 45.1 9.7Ni 148.7 17.4Ga 65.3 3.6Rb 9.1 3.4Sr 395.2 2.2Y 28 23.3 3.2Zr 179 160.0 6.9Hf 4.4 4.2 6.9Nb 19 15.2 4.5Cs 0.13 0.10 9.8Ba 139 129.3 5.0Ta 1.2 1.0 4.2Pb 2.6 2.2 21.8Th 1.1 1.4 7.1U 0.4 23.6La 16 15.0 4.4Ce 39 37.7 4.8Pr 5.4 5.3 5.1Nd 25 23.9 4.9Sm 6.4 6.0 5.1Eu 2.06 2.01 4.0Gd 6.4 6.3 4.8Tb 0.96 0.90 3.2Dy 5.2 5.2 4.8Ho 0.99 0.95 5.1Er 2.5 4.2Tm 0.33 0.32 5.6Yb 2.0 1.9 4.6Lu 0.29 0.26 4.6

RSD = relative standard deviation expressed as a percentage (%).RSD = relative standard deviation expressed as a percentage (%).

affected by F3 recumbent folding (Fig. 3c). These rocks are composedof diopside, epidote, and garnet. Similar calc-silicate layers in theIvisaartoq belt have been ascribed to high temperaturemetasomatismcoeval with the prograde stage of the regional metamorphism (Polatet al., 2007; Ordóñez-Calderón et al., 2008).

3. Petrography

The mineralogical characteristics and interpreted protoliths of thedifferent rock types are presented in Table 1. The primary igneousmineralogy and sedimentary textures are not preserved. These rocksconsist of amphibolite facies metamorphic assemblages with pene-trative foliation.

Hornblende-rich amphibolites (Fig. 4a) are composed of hornble-nde (60–70%), plagioclase (20–35%), quartz (5–10%), and accessory(b2%)minerals such as zircon, apatite, magnetite, and titanite. Garnet,biotite, and cummingtonite are locally present. Biotite generallyreplaces hornblende and garnet.

Cummingtonite-rich amphibolites are rare (Fig. 4b; Table 1). Theyare rich in cummingtonite (40–50%) and poor in hornblende (b5%).

-2, BIR-1, BHVO-1, and BHVO-2.

BIR-1

(%) Recommended Measured (n=5) RSD (%)

47.92 47.92 0.90.96 0.97 1.1

15.50 15.56 1.211.30 11.32 0.80.18 0.17 1.79.70 9.65 0.6

13.30 13.23 0.21.82 1.84 0.40.03 0.03 21.70.02 0.03 18.2

100.26 100.70 0.644.0 43.8 1.118.0 13.3 9.5

BHVO-2

%) Recommended Measured (n=18) RSD (%)

317 308.4 22.6280 213.7 30.445 44.3 7.9119 152.0 9.8

65.2 4.09.8 8.8 5.5389 379.2 9.826 22.7 8.0172 155.7 9.84.1 4.1 8.718 14.5 9.5

0.1 8.9130 130.4 5.41.4 1.0 7.2

1.7 21.91.2 1.8 18.4

0.4 15.915 14.8 5.038 37.3 4.5

5.3 5.125 24.1 4.96.2 6.0 5.2

2.0 2.66.3 6.3 3.50.90 0.90 5.4

5.1 4.41.04 0.94 4.2

2.5 4.60.32 6.2

2 1.9 6.30.28 0.27 5.6

Fig. 5. Zr/Ti versus Nb/Y classification diagram for metavolcanic rocks (amphibolites) inthe Ujarassuit greenstone belt (Table 1). Metasedimentary rocks (biotite schists andquartzitic gneisses) are also plotted for intercomparisons. Compositional fields revisedby Pearce (1996) after Winchester and Floyd (1977).


They containminor amounts of anthophyllite (up to 10%), and abundantquartz (10–15%) and plagioclase (30–50%). Cummingtonite-rich amphi-bolites are spatially associated with meta-ultramafic rocks.

Plagioclase-rich amphibolites are rare (Table 1). They containless hornblende (10–20%), but more plagioclase (60–70%) and quartz(5–15%) than hornblende-rich amphibolites.

Fig. 6. Variation diagrams of Zr versus selected major and trace elements for metavolcanic anmetavolcanic rocks in the Ivisaartoq greenstone belt (see Ordóñez-Calderón et al., 2008).

Meta-ultramafic rocks are represented by serpentinites andactinolite–tremolite-rich amphibolites. Serpentinites are composedof hydrated metamorphic assemblages giving rise to variable miner-alogical compositions (Fig. 4c; Table 1). They include serpentine, talc,magnetite, tremolite, and rarely cummingtonite. Samples 498257 and498262 are composed of weakly serpentinized olivine (Fig. 4d) andclinopyroxene with characteristic mesh texture. Sample 498210 is anactinolite–tremolite-rich amphibolite. Some accessoryminerals (b5%)include hornblende, magnetite, cummingtonite, and rare clinopyrox-ene. This sample is petrographically similar to ultramafic amphibolitesdescribed in the Ivisaartoq greenstone belt (Polat et al., 2007, 2008;Ordóñez-Calderón et al., 2008).

Biotite schists are composed of quartz (15–40%), plagioclase (50–70%), and biotite (10–30%) (Fig. 4e; Table 1). Accessory minerals mayinclude garnet (b3%), epidote, magnetite, and titanite. Some samples(498242 and 498243) contain muscovite (b5%) and microcline (5%).Cummingtonite, anthophyllite, and hornblende occasionally occur inamounts up to 15% (sample 498292 and 498297). Tourmaline (2%)was found only in biotite schists from the Ivisaartoq greenstone belt(Fig. 1).

Quartzitic gneisses were found only in the Ivisaartoq belt (Table 1).They are fine-grained and consist of quartz (70%), plagioclase (10%),biotite (10–15%), andmuscovite (5–20%) (Fig. 4f). Locally, they containgarnet (2%) and hornblende (3%). Accessory minerals include magne-tite, rare tourmaline, and significant amounts of fine-grained zircon.

4. Analytical methods and sampling

Samples were pulverized using an agate mill in the Department ofEarth and Environmental Sciences of the University of Windsor,Canada. Major elements and some trace elements (Sc and Zr) wereanalyzed on a Thermo Jarrel-Ash ENVIRO II ICP-OES in Activationlaboratories Ltd. (ATCLABS) in Ancaster, Canada. The samples were

d metasedimentary rocks (Table 1). Arrows represent the deduced magmatic trends for


mixed with a flux of lithium metaborate and lithium tetraborate, andfused at 1000 °C in an induction furnace. The molten beads wererapidly digested in a solution of 5% HNO3 containing an internalstandard, and mixed continuously until complete dissolution. Loss onignition (LOI) was determined bymeasuring weight loss upon heatingto 1100 °C over a three hour period. Totals ofmajor elements are 100±1wt.% and their analytical precisions are1–2% formostmajor elements(Table 3). The analytical precisions for Sc and Zr are better than 10%.

Transition metals (Ni, Co, Cr, and V), REE, HFSE, and LILE wereanalyzed on a high-sensitivity Thermo Elemental X7 ICP-MS in theGreat Lakes Institute for Environmental Research (GLIER), Uni-versity of Windsor, Canada, following the protocols of Jenner et al.

Fig. 7. Chondrite- and primitive-mantle normalized diagrams for hornblende-rich (Groups 1Amphibolites were divided into groups according to the total trace element abundance, the pnormalization values from Sun and McDonough (1989). Primitive mantle normalization val

(1990). Sample dissolution was conducted under clean lab condi-tions with doubly distilled acids. Approximately 100–130 mg ofsample powder was used for acid digestion. Samples weredissolved in Teflon bombs in a concentrated mixture of HF-HNO3 ata temperature of 120 °C for 3 days and then further attacked with 50%HNO3 until no solid residuewas left. Hawaiian basalt standards BHVO-1and BHVO-2 were used as reference materials to estimate analyticalprecisions (Table 3). Analytical precisions are estimated as follows: 3–10% for REE, Y, Nb, Ta, Rb, Sr, Cs, Ba, and Co; 10–20% for Ni, and Th; and20–30% for U, Pb, V, and Cr.

Major element analyses were recalculated to 100 wt.% anhydrousbasis for inter-comparisons. Chondrite and primitive mantle reservoir

–3) and cummingtonite-rich (Group 4) amphibolites in the Ujarassuit greenstone belt.resence or absence of Nb–Ta anomalies, and the degree of depletion of LREE. Chondriteues and average geochemical composition of modern N-MORB from Hofmann (1988).

Table 4Major (wt.%) and trace element (ppm) concentrations and significant element ratios for metavolcanic rocks.

Group 1 amphibolites Group 2 amphibolites

498237 498259 498271 498273 498274 498278 498281 498206 498211 498218

SiO2(wt.%) 49.3 52.5 47.9 52.5 50.5 51.6 49.2 50.1 47.2 48.1TiO2 0.96 1.00 0.70 0.61 0.76 0.94 1.00 0.66 1.01 0.65Al2O3 15.58 13.57 14.60 13.55 15.30 13.83 14.70 14.90 16.15 16.03Fe2O3 12.73 13.23 12.67 10.08 12.14 13.10 13.11 12.28 15.71 12.94MnO 0.19 0.20 0.20 0.20 0.19 0.20 0.21 0.21 0.24 0.20MgO 7.99 6.84 11.20 7.00 7.99 7.42 7.13 7.13 6.34 8.66CaO 10.72 11.08 10.17 14.30 11.24 11.38 11.88 12.08 10.28 10.28Na2O 2.43 1.23 1.90 1.56 1.63 1.25 2.41 1.81 2.81 2.70K2O 0.06 0.30 0.61 0.19 0.18 0.21 0.26 0.79 0.13 0.35P2O5 0.08 0.08 0.05 0.04 0.06 0.07 0.08 0.05 0.09 0.07LOI (%) 0.66 0.58 1.78 0.58 1.50 0.46 0.53 1.24 0.35 1.08Mg-number (%) 55 51 64 58 57 53 52 54 44 57

Sc (ppm) 38 51 28 35 38 48 39 47 48 43V 266 307 206 227 237 305 282 279 297 240Cr 238 211 196 603 244 257 187 307 143 244Co 46 47 68 48 50 48 52 54 52 48Ni 147 79 433 120 156 89 135 171 78 149Ga 35 33 32 29 34 32 38 41 41 36Rb 2 6 23 4 4 3 5 56 4 7Sr 78 88 65 111 160 72 128 101 94 77Y 19.8 20.9 14.4 13.3 16.3 20.0 21.0 18.7 29.5 19.2Zr 54.2 53.2 44.9 35.3 45.2 58.1 57.1 44.1 60.1 36.3Nb 1.70 2.08 1.49 1.15 1.33 1.96 2.00 0.47 1.21 0.75Cs 0.20 0.17 2.77 0.12 0.22 0.33 0.29 1.30 0.16 0.26Ba 25.38 28.83 57.91 35.85 46.44 21.09 43.51 91.84 31.73 51.91Ta 0.11 0.15 0.08 0.06 0.09 0.12 0.14 0.03 0.08 0.05Pb 1.90 2.64 4.92 5.45 6.84 3.76 5.87 6.09 3.27 5.15Th 0.46 0.41 0.35 0.19 0.19 0.44 0.33 0.08 0.29 0.43U 0.25 0.13 0.11 0.12 0.07 0.14 0.99 0.19 0.08 0.15La 2.68 3.50 2.91 1.91 2.58 3.39 3.21 1.21 2.46 1.59Ce 7.42 9.19 7.23 5.29 7.06 8.68 8.82 3.04 6.70 3.74Pr 1.21 1.35 1.07 0.81 1.04 1.30 1.31 0.54 1.02 0.58Nd 6.11 6.87 5.35 4.18 5.53 6.68 7.02 2.78 5.55 3.08Sm 2.18 2.16 1.61 1.42 1.66 2.19 2.37 1.29 2.05 1.27Eu 0.83 0.82 0.62 0.54 0.60 0.80 0.81 0.47 0.78 0.50Gd 3.02 3.26 2.16 2.00 2.59 3.19 3.19 2.15 3.58 2.07Tb 0.51 0.59 0.39 0.35 0.43 0.53 0.57 0.42 0.67 0.43Dy 3.51 3.68 2.61 2.37 3.00 3.69 3.79 2.97 4.85 3.03Ho 0.77 0.78 0.57 0.53 0.63 0.81 0.83 0.70 1.06 0.72Er 2.23 2.42 1.56 1.53 1.78 2.35 2.40 2.12 3.39 2.18Tm 0.33 0.36 0.23 0.21 0.27 0.34 0.35 0.32 0.51 0.32Yb 2.15 2.39 1.58 1.40 1.73 2.14 2.26 2.14 3.31 2.33Lu 0.32 0.36 0.22 0.22 0.27 0.33 0.33 0.32 0.53 0.35

La/Ybcn 0.84 0.99 1.24 0.92 1.00 1.06 0.96 0.38 0.50 0.46La/Smcn 0.77 1.02 1.14 0.85 0.98 0.97 0.85 0.59 0.75 0.79Gd/Ybcn 1.14 1.10 1.11 1.16 1.21 1.20 1.14 0.81 0.87 0.72(Eu/Eu⁎)cn 0.98 0.94 1.02 0.98 0.88 0.92 0.91 0.87 0.88 0.94(Ce/Ce⁎)cn 0.99 1.02 0.98 1.02 1.04 0.99 1.04 0.91 1.02 0.94Al2O3/TiO2 16.19 13.60 20.97 22.37 20.01 14.70 14.69 22.50 15.91 24.78Nb/Ta 15.75 13.84 18.69 18.52 15.63 16.18 14.80 15.29 15.07 14.68Zr/Y 2.74 2.55 3.12 2.66 2.77 2.90 2.72 2.36 2.03 1.89Ti/Zr 106.36 112.55 92.92 102.77 101.37 97.18 105.07 89.92 101.20 106.91(Nb/Nb⁎)pm 0.63 0.71 0.60 0.77 0.78 0.65 0.79 0.62 0.58 0.37(Zr/Zr⁎)pm 1.04 0.97 1.07 1.01 1.04 1.06 0.98 1.63 1.25 1.28(Ti/Ti⁎)pm 0.92 0.87 0.88 0.86 0.86 0.86 0.87 0.76 0.72 0.72ΣREE 33.26 37.72 28.11 22.75 29.18 36.42 37.27 20.47 36.46 22.19

North 64°53.566′ 64°55.080′ 64°51.480′ 64°53.002′ 64°52.855′ 64°53.398′ 64°50.941′ 64°50.962′ 64°50.885′ 64°50.202′West 50°12.867′ 50°12.317′ 49°59.689′ 49°56.825′ 49°57.233′ 50°1.744′ 49°59.203′ 50°12.649′ 50°12.389′ 50°12.887′

LDL = lower than detection limit.Metavolcanic rocks with sample number b498270 belong to the Western flank of the Ujarassuit greenstone belt.Metavolcanic rocks with sample number N498270 belong to the Eastern flank of the Ujarassuit greenstone belt.


compositions are those of Sun and McDonough (1989) and Hofmann(1988), respectively. The Eu (Eu/Eu⁎), Ce (Ce/Ce⁎), Nb (Nb/Nb⁎), Ti (Ti/Ti⁎), and Zr (Zr/Zr⁎) anomalies were calculated with respect to theneighboring immobile elements, following the method of Taylor andMcLennan(1985).Mg-numbers (%)were calculatedas themolecular ratioof Mg2+/(Mg2++Fe2+) where Fe2+ is assumed to be 90% of the total Fe.

5. Geochemistry

5.1. Amphibolites

Most amphibolites show a basaltic composition on the Zr/Ti versusNb/Y diagram (Figs. 5 and 6). They have been subdivided into four

Group 2 amphibolites Group 3 amphibolites Group 4 amphibolites

498219 498224 498227 498241 498244 498256B 498229 498235 498260 498261 498265 498228 498239

49.3 48.6 54.0 54.8 50.1 49.2 48.1 51.2 49.0 50.5 49.0 53.3 53.70.72 0.62 0.65 0.45 0.66 0.40 0.80 0.79 1.01 1.20 1.00 0.32 0.3314.93 15.80 13.09 16.27 15.87 13.81 16.24 14.60 15.62 15.54 15.55 15.19 15.8313.26 12.04 9.85 11.68 11.31 11.12 12.70 11.31 12.50 12.75 13.37 10.45 10.570.21 0.21 0.21 0.14 0.20 0.21 0.22 0.21 0.20 0.20 0.19 0.21 0.148.44 7.88 6.61 7.36 5.55 11.26 8.39 6.26 8.29 6.33 9.34 10.75 10.8510.37 12.47 14.07 7.72 13.63 12.47 10.62 13.90 10.81 10.41 9.09 7.58 7.172.30 1.84 1.30 1.50 2.62 1.32 2.79 1.30 2.39 2.51 2.16 1.87 1.330.44 0.50 0.19 0.02 0.06 0.14 0.13 0.34 0.14 0.41 0.18 0.32 0.020.05 0.06 0.05 0.04 0.05 0.03 0.06 0.06 0.08 0.10 0.07 0.04 0.040.97 1.01 1.84 LDL 2.87 1.21 1.28 1.20 0.28 0.07 1.37 2.68 0.4456 56 57 56 49 67 57 52 57 50 58 67 67

54 44 40 52 47 54 40 50 42 45 41 46 45285 251 244 292 234 224 247 285 264 323 277 223 232301 263 351 37 238 618 263 319 251 219 238 456 37945 53 39 50 58 57 54 45 51 45 48 47 50100 150 138 59 161 159 190 138 166 100 155 124 12536 32 29 30 30 25 34 33 37 55 36 29 2711 18 6 1 1 4 5 12 1 6 11 10 283 87 47 36 94 37 69 50 136 104 44 74 9121.4 19.5 15.2 17.4 19.5 12.5 16.4 18.9 20.0 23.6 20.3 12.6 11.931.5 31.5 30.6 23.0 36.9 18.2 45.7 42.5 57.3 67.2 55.4 22.5 22.00.77 0.59 0.78 0.68 0.69 0.31 1.35 1.39 1.77 2.54 1.79 0.46 0.510.48 0.46 0.22 0.07 0.10 0.42 0.21 0.75 0.11 0.18 1.39 1.27 0.4055.65 36.29 33.84 11.62 13.90 18.10 32.57 17.95 41.61 173.98 38.00 50.48 18.070.06 0.05 0.06 0.04 0.03 0.02 0.09 0.07 0.12 0.17 0.13 0.03 0.036.22 5.29 1.54 2.05 2.73 2.37 7.40 6.57 2.68 6.20 1.84 12.71 11.220.43 0.10 0.16 0.13 0.22 0.05 0.11 0.15 0.24 0.32 0.26 0.10 0.290.24 0.11 0.18 0.20 0.48 0.04 0.23 0.07 0.08 0.11 0.13 0.36 0.232.20 1.89 1.57 1.50 1.45 0.68 2.14 1.72 2.55 3.37 2.42 2.06 1.885.20 4.74 4.32 4.30 3.91 1.84 6.15 4.58 7.31 9.57 7.18 4.19 3.710.74 0.62 0.66 0.70 0.61 0.29 1.00 0.74 1.15 1.53 1.11 0.55 0.433.66 3.58 3.30 3.15 3.22 1.72 4.85 4.02 6.17 7.75 6.06 2.52 1.941.36 1.28 1.31 0.92 1.32 0.80 1.77 1.56 1.99 2.53 2.08 0.68 0.570.55 0.48 0.54 0.26 0.60 0.24 0.68 0.61 0.68 0.97 0.80 0.31 0.282.35 2.28 2.04 1.71 2.26 1.42 2.43 2.50 3.12 3.65 3.09 1.30 1.160.49 0.45 0.39 0.36 0.47 0.28 0.44 0.46 0.53 0.62 0.54 0.24 0.243.54 3.18 2.61 2.72 3.17 2.12 2.88 3.10 3.59 4.29 3.58 1.97 1.850.82 0.73 0.59 0.62 0.73 0.47 0.60 0.72 0.76 0.94 0.80 0.44 0.442.45 2.23 1.71 2.03 2.24 1.41 1.85 2.07 2.26 2.69 2.25 1.43 1.410.37 0.32 0.24 0.30 0.33 0.22 0.27 0.30 0.33 0.39 0.33 0.22 0.212.45 2.22 1.73 2.05 2.30 1.43 1.80 2.11 2.07 2.50 2.23 1.52 1.550.38 0.35 0.26 0.32 0.35 0.22 0.26 0.33 0.33 0.38 0.32 0.25 0.25

0.60 0.57 0.61 0.49 0.43 0.32 0.80 0.55 0.83 0.91 0.73 0.91 0.821.02 0.92 0.75 1.02 0.69 0.53 0.76 0.69 0.80 0.84 0.73 1.91 2.080.77 0.83 0.95 0.68 0.79 0.80 1.10 0.96 1.22 1.18 1.12 0.70 0.610.94 0.85 1.00 0.63 1.06 0.68 1.01 0.95 0.84 0.98 0.96 1.01 1.070.98 1.05 1.02 1.01 1.00 1.00 1.01 0.98 1.03 1.01 1.06 0.94 0.9920.73 25.39 20.24 35.97 23.93 34.71 20.38 18.51 15.47 12.90 15.55 47.32 47.3413.36 12.43 13.70 16.45 20.10 16.71 14.80 20.55 15.11 14.76 14.15 16.10 18.891.47 1.62 2.01 1.33 1.89 1.46 2.79 2.25 2.87 2.85 2.74 1.79 1.84137.11 118.35 126.89 117.81 107.74 130.89 104.45 111.19 105.70 107.46 108.13 85.56 91.010.32 0.56 0.63 0.62 0.49 0.67 1.15 1.11 0.93 1.00 0.92 0.42 0.280.99 1.03 1.03 0.94 1.25 1.09 1.09 1.19 1.14 1.06 1.09 1.20 1.470.70 0.67 0.82 0.59 0.69 0.66 0.91 0.84 0.94 0.94 0.92 0.59 0.6426.54 24.34 21.26 20.92 22.96 13.14 27.12 24.83 32.85 41.21 32.79 17.68 15.92

64°51.000′ 64°49.455′ 64°51.388′ 64°52.912′ 64°52.715′ 64°54.985′ 64°51.485′ 64°51.711′ 64°54.142′ 64°53.816′ 64°53.785′ 64°51.483′ 64°52.959′50°11.826′ 50°13.696′ 50°11.513′ 50°13.083′ 50°13.213′ 50°12.391′ 50°11.446′ 50°11.921′ 50°13.996′ 50°15.656′ 50°15.157′ 50°11.420′ 50°13.087′

(continued on next page)


major groups on the basis of their chondrite- and primitive mantle-normalized trace element patterns (Fig. 7; Table 4). Groups 1, 2, and 3are hornblende-rich amphibolites. They show similar mineralogicalcomposition and can only be distinguished on the basis of theirtrace element characteristics. In contrast, Group 4 amphibolites arecummingtonite-rich amphibolites (Table 1).

Hornblende-rich amphibolites (Groups 1–3) possess Zr/Y ratios(Zr/Y=1.5–4.6) similar to those of modern tholeiitic basalts (Zr/Y=1.3–3.1) (see Barrett and MacLean, 1994). They have Mg-numbersranging from 44 to 67 and variable concentrations of SiO2 (47.2–54.8 wt.%), TiO2 (0.4–1.2 wt.%), Fe2O3 (9.9–15.7 wt.%), Al2O3 (13.1–16.3 wt.%), MgO (5.6–11.3 wt.%), Zr (18–67 ppm), and REE (13.1–

Table 4 (continued )

Group 4 amphibolites Plagioclase-rich amphibolites Serpentinite rocks and actinolite–tremolite-rich amphibolite⁎

498248 498264 498209 498279 498210⁎ 498220 498240 498246 498247 498257 498262

SiO2(wt.%) 52.0 50.9 67.4 55.3 48.5 47.9 47.5 44.7 44.5 42.3 43.6TiO2 0.33 0.26 0.57 1.00 0.40 0.11 0.11 0.42 0.20 0.36 0.17Al2O3 14.63 18.61 14.75 10.59 9.43 4.72 4.30 6.88 5.14 3.84 4.41Fe2O3 10.81 6.86 3.47 13.42 11.14 11.00 8.32 13.37 11.47 15.51 14.72MnO 0.17 0.13 0.05 0.25 0.16 0.15 0.11 0.18 0.11 0.18 0.22MgO 13.06 10.84 2.83 6.34 19.73 33.87 39.14 31.76 36.67 34.51 34.06CaO 7.32 9.47 6.70 11.29 9.53 1.91 0.49 2.46 1.80 2.85 2.45Na2O 1.63 2.63 3.82 0.95 0.91 0.23 0.05 0.17 0.07 0.27 0.28K2O 0.04 0.23 0.16 0.74 0.20 0.08 LDL 0.05 LDL 0.08 0.03P2O5 0.03 0.04 0.21 0.12 0.03 0.03 0.02 0.04 0.02 0.07 0.01LOI (%) 1.48 1.50 0.34 0.98 3.82 4.65 3.24 4.02 10.96 9.51 5.88Mg-number (%) 71 76 62 48 78 86 90 82 86 82 82

Sc (ppm) 41 27 11 18 35 20 12 19 16 13 14V 233 124 86 152 178 117 74 148 102 101 86Cr 765 99 132 229 2377 4141 6312 5173 5216 2543 2634Co 57 44 16 56 74 108 113 111 103 117 130Ni 178 167 63 130 658 997 2212 1311 1403 1082 1400Ga 26 28 47 35 16 9 7 14 9 9 8Rb 2 12 4 11 1 3 0 1 0 1 0Sr 67 106 372 411 38 16 1 11 2 6 6Y 12.1 7.3 9.2 14.7 8.0 3.2 2.4 5.5 4.2 5.9 2.5Zr 20.3 28.6 145.7 86.6 20.72 4.19 6.17 19.84 7.84 21.04 8.49Nb 0.36 0.83 4.92 5.20 0.65 0.18 0.17 0.84 0.28 0.67 0.35Cs 0.53 1.75 0.98 0.31 0.11 1.88 0.05 0.23 1.14 0.20 0.11Ba 28.51 27.69 127.10 63.15 4.30 2.86 0.65 2.52 0.39 1.05 2.00Ta 0.02 0.04 0.35 0.33 0.04 0.01 0.02 0.05 0.02 0.04 0.02Pb 6.91 4.77 4.89 11.29 5.08 5.17 2.58 4.34 2.26 1.82 2.50Th 0.14 0.37 4.80 1.23 0.11 0.09 0.08 0.24 0.11 0.33 0.10U 0.16 0.10 1.07 0.96 0.12 0.05 0.04 0.08 0.05 0.18 0.03La 1.78 3.33 18.18 11.85 0.75 0.35 0.16 1.18 0.68 2.30 0.56Ce 3.75 6.12 47.04 33.16 2.41 0.91 0.42 2.99 1.91 5.85 1.48Pr 0.48 0.83 5.05 4.51 0.36 0.09 0.05 0.41 0.24 0.74 0.17Nd 2.18 3.61 19.35 19.97 2.10 0.48 0.26 1.79 1.11 3.27 0.80Sm 0.68 0.86 3.56 4.20 0.73 0.13 0.07 0.52 0.38 0.79 0.20Eu 0.34 0.36 1.00 1.18 0.19 0.04 0.01 0.14 0.06 0.30 0.05Gd 1.04 1.08 3.02 4.10 1.08 0.33 0.19 0.63 0.59 1.02 0.31Tb 0.23 0.17 0.35 0.56 0.20 0.06 0.04 0.12 0.09 0.16 0.06Dy 1.87 1.19 1.92 3.04 1.35 0.47 0.38 0.94 0.67 1.10 0.46Ho 0.44 0.27 0.33 0.57 0.31 0.12 0.10 0.20 0.16 0.22 0.09Er 1.40 0.84 0.95 1.49 0.92 0.38 0.32 0.64 0.52 0.65 0.29Tm 0.20 0.13 0.12 0.22 0.13 0.06 0.05 0.10 0.07 0.10 0.05Yb 1.48 0.87 0.78 1.27 0.94 0.45 0.33 0.72 0.52 0.62 0.28Lu 0.23 0.14 0.12 0.20 0.14 0.07 0.05 0.10 0.08 0.10 0.05

La/Ybcn 0.81 2.58 15.64 6.29 0.54 0.53 0.31 1.10 0.88 2.48 1.35La/Smcn 1.64 2.42 3.21 1.78 0.65 1.73 1.45 1.43 1.13 1.83 1.75Gd/Ybcn 0.57 1.01 3.12 2.61 0.94 0.59 0.47 0.71 0.91 1.32 0.91(Eu/Eu⁎)cn 1.24 1.15 0.93 0.87 0.64 0.53 0.34 0.75 0.39 1.03 0.62(Ce/Ce⁎)cn 0.97 0.89 1.18 1.09 1.11 1.23 1.11 1.04 1.12 1.08 1.16Al2O3/TiO2 44.41 71.25 25.70 10.55 23.45 44.55 40.58 16.52 26.08 10.78 26.00Nb/Ta 16.81 20.77 14.25 15.73 16.54 12.85 8.40 17.97 17.66 17.51 14.25Zr/Y 1.69 3.93 15.87 5.90 2.59 1.32 2.59 3.63 1.85 3.59 3.33Ti/Zr 97.12 54.81 23.61 69.50 116.30 151.37 102.91 125.89 150.73 101.60 119.90(Nb/Nb⁎)pm 0.29 0.30 0.21 0.55 0.94 0.41 0.64 0.65 0.42 0.31 0.60(Zr/Zr⁎)pm 1.17 1.13 1.23 0.66 1.17 1.19 3.29 1.45 0.84 0.92 1.48(Ti/Ti⁎)pm 0.64 0.74 0.89 0.98 0.99 0.80 1.09 1.59 1.01 1.10 1.27ΣREE 16.11 19.80 101.76 86.33 11.60 3.93 2.44 10.48 7.08 17.21 4.86

North 64°54.043′ 64°53.799′ 64°50.855′ 64°53.795′ 64°50.624′ 64°50.813′ 64°52.959′ 64°52.654′ 64°52.668′ 64°54.983′ 64°53.816′West 50°13.027′ 50°15.778′ 50°12.627′ 50°03.025′ 50°12.603′ 50°12.733′ 50°13.087′ 50°12.887′ 50°12.784′ 50°12.314′ 50°15.656′


41.2 ppm) (Fig. 6; Table 4). Groups 1–3 amphibolites both in theUjarassuit belt and in the Ivisaartoq belt (Ordóñez-Calderón et al.,2008), display collinear trends for major elements, HFSE, REE, andtransition metals on variation diagrams of Zr indicating comparablecompositions (Table 4). However, in the Ujarassuit belt they haveslightly lower concentrations of Ni (59–190 ppm) than those in theIvisaartoq belt (74–279 ppm) (see Ordóñez-Calderón et al., 2008). Theyshow variable Eu (Eu/Eu⁎=0.63–1.06), Zr (Zr/Zr⁎=0.94–1.63), and Ti(Ti/Ti⁎=0.59–0.94) anomalies on chondrite- and primitive mantle-normalized diagrams (Fig. 7).

Group 1 amphibolites occur in the eastern and western flanksof the Ujarassuit greenstone belt (Fig. 7a–b). This amphibolitegroup displays near-flat REE patterns (La/Smcn=0.77–1.14; Gd/Ybcn=1.10–1.21) and negative Nb anomalies (Nb/Nb⁎=0.60–0.79).These trace element characteristics are similar to those shown bywell preserved pillow basalts in the Ivisaartoq greenstone belt(Polat et al., 2007). In addition, Group 1 amphibolites in this studyare compositionally similar to the least altered Group 1 amphibo-lites in the Ivisaartoq greenstone belt (Ordóñez-Calderón et al.,2008).


Group 2 amphibolites occur in the western flank of the Ujarassuitgreenstone belt (Fig. 7c–d). The trace element patterns of this group ofamphibolites are characterized by depleted LREE (La/Smcn=0.53–1.02)and slightly fractionated HREE (Gd/Ybcn=0.68–0.95) patterns. Inaddition, they show pronounced negative Nb (Nb/Nb⁎=0.32–0.67)anomalies. The trace element patterns of Group 2 amphibolites arecomparable to those of Group 3 amphibolites in the Ivisaartoqgreenstone belt (Ordóñez-Calderón et al., 2008) and amphibolites inthe Qussuk greenstone belt in the Akia terrane (Garde, 2007).

Group 3 amphibolites occur in the western flank of the Ujarassuitbelt (Fig. 1). They exhibit depleted LREE patterns (La/Smcn=0.69–0.84;Gd/Ybcn=0.96–1.22) and lack significant Nb (Nb/Nb⁎=0.92–1.15)anomalies (Fig. 7e–f). The trace element patterns of this group ofamphibolites are comparable to those of average modern N-MORB(Hofmann, 1988).

Relative to Groups 1–3 amphibolites, cummingtonite-rich amphi-bolites (Group 4) have higher SiO2 (50.9–53.7 wt.%), MgO (10.8–13.1 wt.%), and Mg-numbers (67–76) (Table 4), and lower CaO (7.2–9.5wt.%), Fe2O3 (6.9–10.8wt.%), TiO2 (0.26–0.33wt.%), Zr (20–29ppm),and REE (15.9–19.8 ppm) (Tables 4 and 6). They possess high Ni (124–178 ppm), Cr (99–765 ppm), and Sc (27–46 ppm) contents. Group 4amphibolites possess higher Al2O3/TiO2 (44–71), lower Ti/Zr (55–97),and comparable Nb/Ta (14–21) ratios with respect to Group 1–3amphibolites (Table 6). They display enriched LREE (La/Smcn=1.64–2.42) patterns and flat to fractionated HREE (Gd/Ybcn=0.57–1.01)patterns (Fig. 7g–h). In addition, Group 4 amphibolites havepronounced negative Nb (Nb/Nb⁎=0.28–0.42) and Ti (Ti/Ti⁎=0.59–0.74) anomalies, andminor positive Eu (Eu/Eu⁎=1.01–1.24) anomalies.

Plagioclase-rich amphibolites have higher contents of SiO2 (55.3–67.4wt.%) thanGroup1–4amphibolites (Tables4 and6).OnZr/Ti versusNb/Y diagram they straddle the field of basalts and andesites (Fig. 5). Inaddition, they exhibit Zr/Y (5.9–15.9) ratios comparable to those oftransitional (Zr/Y=4.5–7.0) and calc-alkaline (Zr/YN7.0) volcanic rocks(see Barrett and MacLean, 1994). They have variable Mg-numbers (48–62) and Zr (87–146 ppm) contents (Table 4). Relative to Groups 1–3amphibolites, they possess comparable concentrations of TiO2 (0.57–

Fig. 8. Chondrite- and primitive-mantle normalized diagrams for (a–b) Plagioclase-rich amultramafic rocks (Table 1). Normalization values as in Fig. 7.

1.0 wt.%), lower Fe2O3 (3.5–13.4 wt.%), HREE (7.6–11.5 ppm), Sc (11–18 ppm), and V (86–152 ppm), and higher contents of Th (1.2–4.8 ppm)and LREE (74.9–94.2 ppm). On chondrite- and primitive mantle-normalized diagrams (Fig. 8a–b), they exhibit fractionated LREE andHREEpatterns (La/Smcn=1.78–3.21;Gd/Ybcn=2.61–3.12), pronouncednegative Nb anomalies (Nb/Nb⁎=0.21–0.55), and small negative Euanomalies (Eu/Eu⁎=0.87–0.93).

5.2. Meta-ultramafic rocks

In the Ujarassuit greenstone belt, an actinolite–tremolite-richamphibolite (sample 498210; Table 4) shows high concentrations ofMgO (19.7 wt.%), CaO (9.5 wt.%), and Fe2O3 (11.1 wt.%), and low TiO2

(0.4 wt.%) and Zr (20.72 wt.%) contents. This ultramafic rock shows adepleted LREEpattern (La/Smcn=0.65), nearflatHREE (Gd/Ybcn=0.94),and absence of a significant Nb (Nb/Nb⁎=0.94) anomaly.

The serpentinites are characterized by high loss-on-ignition (LOI=3.2–10.9 wt.%) values, high concentrations of MgO (31.8–39.1 wt.%), Ni(997–2212 ppm), Cr (2542–6312 ppm), and Co (103–130 ppm), and lowconcentrations of TiO2 (0.11–0.42 wt.%) and Zr (4.2–20.7 ppm) (Tables 1and 4). Relative to the actinolite–tremolite-rich amphibolites, they havelower concentrations of CaO (0.5–2.9 wt.%), Al2O3, (3.8–6.9 wt.%), andREE (2.4–17.2 ppm), and higherMgO andNi contents (Table 6; Fig. 8c–d).On chondrite- and primitive mantle-normalized diagrams, they displayvariably enriched LREE (La/Smcn=1.13–1.83) and fractionated to flatHREE (Gd/Ybcn=0.47–1.32) patterns. Serpentinites show negative Nbanomalies (Nb/Nb⁎=0.31–0.65)andvariableEu(Eu/Eu⁎=0.34–1.03), Zr(Zr/Zr⁎=0.84–3.29), and Ti (Ti/Ti⁎=0.8–1.59) anomalies.

5.3. Biotite schists and quartzitic gneisses

Biotite schists have large variations of SiO2 (53.2–72.4 wt.%), MgO(1.3–8.4 wt.%), Fe2O3 (2.5–10.2 wt.%), TiO2 (0.32–0.92 wt.%), andAl2O3 (12.6–24.0 wt.%) contents (Fig. 6) (Table 5). Niobium and REE(54–342 ppm) display positive correlations with Zr (108–253 ppm)(Fig. 6). They have relatively high concentrations of transition metals

phibolites with basaltic andesite and andesite geochemical composition. (c–d) Meta-

Table 5Major (wt.%) and trace element (ppm) concentrations and significant element ratios for metasedimentary rocks.

Ujarassuit belt, Western flank Ivisaartoq belt, lower metasedimentary unit Ivisaartoq belt, lower metasedimentary unit

Biotite schists Biotite schists Quartzitic gneisses

498226 498230 498231 498242 498243 498292 498293 498295 498297 496101 498294 498296 498299

SiO2 (wt.%) 62.8 53.1 69.2 72.4 71.5 58.1 71.2 70.7 59.9 77.7 76.3 72.0 81.6TiO2 0.50 0.50 0.36 0.32 0.35 0.92 0.82 0.80 0.63 0.28 0.32 0.61 0.16Al2O3 16.15 24.04 15.42 14.51 14.81 15.29 12.83 12.59 16.38 11.73 11.42 10.89 11.09Fe2O3 6.36 4.60 2.82 2.53 2.54 10.17 7.13 6.95 6.89 2.83 5.12 8.02 1.41MnO 0.12 0.07 0.04 0.04 0.04 0.17 0.10 0.09 0.14 0.05 0.05 0.10 0.02MgO 3.75 4.23 1.25 1.34 1.62 8.40 2.44 3.11 4.23 0.62 0.75 1.58 0.46CaO 4.09 7.70 3.82 2.80 2.84 3.56 1.29 1.39 5.11 1.92 0.90 3.13 0.69Na2O 3.16 4.71 3.97 2.96 2.94 2.67 2.36 2.66 4.86 2.57 3.29 2.50 1.63K2O 2.95 0.99 2.97 2.99 3.20 0.61 1.68 1.57 1.66 2.22 1.79 0.97 2.88P2O5 0.12 0.02 0.14 0.14 0.13 0.11 0.13 0.11 0.20 0.04 0.04 0.15 0.02LOI (%) 1.67 2.49 1.99 1.33 1.27 0.83 1.49 1.23 1.21 0.98 0.85 0.46 1.17Mg-number (%) 54 65 47 51 56 62 40 47 55 30 22 28 39CIA (%) 51 51 49 53 46 58 62 60 47 54 56 51 61

Sc (ppm) 16 13 11 7 10 37 17 21 18 4 7 12 2V 110 57 71 49 64 187 117 125 134 LDL LDL LDL LDLCr 115 141 94 51 73 753 121 129 131 LDL LDL LDL LDLCo 24 20 21 14 31 52 19 19 25 1 1 5 1Ni 75 114 272 57 108 328 67 53 93 7 5 9 4Ga 68 47 120 150 138 54 81 75 155 88 93 76 74Rb 381 76 115 74 83 16 49 48 49 57 48 20 77Sr 205 378 136 281 214 334 106 95 175 37 40 50 25Y 12.0 9.3 11.0 7.1 7.8 16.4 26.8 33.4 12.2 140.4 103.9 143.6 102.3Zr 108 253 115 126 113 166 200 237 122 618 702 438 237Nb 2.78 3.06 4.09 3.94 3.38 6.24 10.89 10.89 3.58 12.22 19.89 13.08 15.25Cs 13.01 2.12 0.87 2.82 3.25 2.00 5.07 6.25 4.10 2.95 5.65 2.07 4.68Ba 272 156 654 886 786 168 346 295 922 278 303 230 266Ta 0.19 0.17 0.25 0.27 0.21 0.42 0.71 0.72 0.23 0.87 0.70 0.90 1.04Pb 19.37 32.71 31.62 37.66 56.11 12.00 12.67 10.15 46.06 7.60 14.58 4.95 11.77Th 3.24 26.88 9.63 13.51 9.83 2.52 4.32 3.80 7.03 4.51 7.86 4.99 7.84U 1.07 1.92 2.23 2.62 2.09 0.59 14.63 0.95 0.96 1.00 1.50 0.96 1.44La 12.53 100.07 43.87 40.53 31.36 14.85 17.38 21.31 38.37 29.93 56.96 39.08 21.02Ce 20.91 155.26 71.33 73.91 57.21 34.45 40.61 48.66 76.47 78.51 130.78 88.34 137.97Pr 2.33 17.00 7.85 7.28 5.92 4.58 5.04 6.26 8.43 11.13 19.03 12.95 9.04Nd 8.86 53.40 26.82 23.70 19.57 19.43 21.57 26.96 31.40 54.02 86.48 60.36 39.15Sm 1.97 6.24 4.03 3.23 2.95 4.80 4.87 6.64 4.82 17.00 24.30 17.81 11.46Eu 0.58 1.29 0.90 0.75 0.69 1.46 0.97 1.26 1.14 3.13 3.33 3.60 1.79Gd 2.15 4.40 3.24 2.35 2.24 4.59 4.77 6.69 3.93 22.52 27.38 23.94 12.43Tb 0.32 0.45 0.40 0.27 0.31 0.63 0.73 0.96 0.46 4.10 4.13 4.10 2.51Dy 1.90 2.02 2.12 1.33 1.55 3.51 4.80 6.23 2.50 27.18 23.35 26.90 18.37Ho 0.39 0.34 0.38 0.24 0.30 0.69 1.15 1.36 0.47 5.85 4.37 5.65 4.20Er 1.14 0.92 1.08 0.71 0.80 1.99 3.80 4.54 1.32 17.89 12.37 16.40 13.36Tm 0.16 0.13 0.15 0.11 0.10 0.32 0.62 0.72 0.17 2.67 1.83 2.52 1.93Yb 1.02 0.77 0.91 0.59 0.69 2.57 4.89 5.29 1.09 17.90 12.64 16.70 12.60Lu 0.16 0.15 0.15 0.09 0.10 0.40 0.77 0.86 0.16 2.61 1.98 2.45 1.83

La/Ybcn 8.31 87.95 32.61 46.04 30.52 3.89 2.39 2.71 23.69 1.13 3.03 1.58 1.12La/Smcn 3.99 10.08 6.83 7.89 6.68 1.94 2.24 2.02 5.01 1.11 1.47 1.38 1.15Gd/Ybcn 1.71 4.64 2.89 3.21 2.62 1.44 0.79 1.02 2.91 1.02 1.75 1.16 0.80(Eu/Eu⁎)cn 0.86 0.75 0.76 0.84 0.82 0.95 0.62 0.58 0.80 0.49 0.40 0.53 0.46(Ce/Ce⁎)cn 0.93 0.91 0.93 1.04 1.01 1.01 1.05 1.02 1.02 1.04 0.96 0.95 2.41Al2O3/TiO2 32.04 48.13 42.92 45.54 41.86 16.60 15.69 15.84 26.10 42.01 36.18 17.92 70.58Nb/Ta 14.46 17.97 16.05 14.39 16.11 14.92 15.39 15.03 15.81 14.06 28.50 14.58 14.70Zr/Y 8.99 27.23 10.49 17.79 14.60 10.10 7.44 7.11 10.03 4.40 6.76 3.05 2.31Ti/Zr 28.00 11.82 18.73 15.18 18.73 33.25 24.57 20.10 30.77 2.71 2.70 8.32 3.98(Nb/Nb⁎)pm 0.18 0.02 0.08 0.07 0.08 0.41 0.51 0.49 0.09 0.43 0.38 0.38 0.48(Zr/Zr⁎)pm 1.81 0.97 0.77 1.01 1.04 1.20 1.36 1.24 0.70 1.43 1.07 0.93 0.78(Ti/Ti⁎)pm 0.82 0.67 0.50 0.68 0.65 0.79 0.56 0.42 0.75 0.03 0.04 0.07 0.03ΣREE 54.43 342.43 163.23 155.10 123.80 94.27 111.98 137.73 170.72 294.43 408.95 320.82 287.67

North 64°51.359′ 64°51.446′ 64°51.446′ 64°52.912′ 64°52.912′ 64°43.597′ 64°43.597′ 64°43.394′ 64°43.527′ 64°43.420′ 64°43.597′ 64°43.568′ 64°43.391′West 50°11.553′ 50°11.409′ 50°11.409′ 50°13.083′ 50°13.083′ 49°58.033′ 49°58.033′ 50°00.442′ 49°59.737′ 50°00.013′ 49°59.045′ 49°59.516′ 50°00.856′

LDL = lower than detection limit.The chemical index of alteration CIA=100(Al2O3/[Al2O3+Na2O+K2O+CaO⁎]) is calculated in molar proportions where CaO⁎ represents CaO only in silicates (see Nesbitt and Young,1984; Fedo et al., 1995).


Ni (53–328 ppm), Sc (7–37 ppm), Cr (51–753 ppm), and V (49–187 ppm), which are comparable to those of Groups 1–3 amphibo-lites (Tables 5 and 6). On the Zr/Ti versus Nb/Y diagram, they plot inthe field of andesites and basaltic andesites (Fig. 5). On chondrite- andprimitive mantle-normalized diagrams, they display fractionated

REE patterns with high La/Smcn (1.94–10.08) and Gd/Ybcn (0.79–4.64)ratios (Fig. 9a–d). In addition, they display pronounced negative Nb(Nb/Nb⁎=0.02–0.51), Eu (Eu/Eu⁎=0.58;–0.95), and Ti (Ti/Ti⁎=0.42–0.82) anomalies, and negative to positive Zr (Zr/Zr⁎=0.70–1.81)anomalies.

Table 6Summary of significant geochemical characteristics and trace element ratios for metavolcanic and metasedimentary rocks.

Group 1amphibolites

Group 2amphibolites

Group 3amphibolites

Group 4amphibolites

Plagioclase-richamphibolites

Serpentinites Actinolite–tremolite-rich amphibolitesa

Biotiteschists

Quartziticgneisses

SiO2 (wt.%) 48–53 47–55 48–51 51–54 55–67 42–48 46–53 53–72 72–82TiO2 0.6–1.0 0.4–1.0 0.8–1.2 0.26–0.33 0.6–1.0 0.1–0.4 0.2–0.4 0.3–0.9 0.2–0.6Al2O3 13.6–15.6 13.1–16.3 14.6–16.2 14.6–18.6 10.6–14.8 3.8–6.9 5.6–12.7 12.6–24.0 10.9–11.7Fe2O3 10.1–13.2 9.9–15.7 11.3–13.4 6.9–10.8 3.5–13.4 8.3–15.5 9.0–11.6 2.5–10.2 1.4–8.0MgO 6.8–11.2 5.6–11.3 6.3–9.3 10.8–13.1 2.8–6.3 31.8–39.1 15.5–21.8 1.3–8.4 0.5–1.6CaO 10.2–14.3 7.7–14.1 9.1–10.8 7.2–9.5 6.7–11.3 0.5–2.9 9.4–11.4 1.3–7.7 0.7–3.1

Zr (ppm) 35–58 18–60 43–67 20–29 87–146 4–21 11–33 108–253 237–702Ni 79–433 59–171 100–190 124–178 63–130 1000–2200 469–1200 53–328 4.1–8.7Cr 187–603 37–618 219–319 99–765 132–229 2500–6300 1800–12000 51–753 b 30ΣREE 23–38 13–37 25–41 16–20 86–102 2.4–17.2 12–25 54–342 288–409

Al2O3/TiO2 14–22 16–36 13–20 44–71 10–26 11–45 23–29 17–48 18–71Ti/Zr 93–105 90–137 105–111 55–97 24–70 102–151 79–108 12–30 2.7–8.3Nb/Ta 14–19 12–20 14–21 16–20 14–16 8–18 11–15 14–18 14–29Zr/Y 2.6–3.1 1.3–2.4 2.3–2.9 1.7–3.9 5.9–15.9 1.3–3.6 2.3–3.2 7.1–27.2 2.3–6.8

La/Ybcn 0.84–1.24 0.32–0.61 0.55–0.91 0.81–2.58 6.29–15.64 0.31–2.48 1.7–3.3 2.39–87.95 1.12–3.03La/Smcn 0.77–1.14 0.53–1.02 0.69–0.84 1.64–2.42 1.78–3.21 1.13–1.83 1.6–3.4 1.94–10.08 1.11–1.47Gd/Ybcn 1.10–1.21 0.68–0.95 0.96–1.22 0.57–1.01 2.61–3.12 0.47–1.32 0.8–1.2 0.79–4.64 0.80–1.75

(Eu/Eu*)cn 0.88–1.02 0.63–1.06 0.84–1.01 1.01–1.24 0.87–0.93 0.34–1.03 0.71–1.37 0.58–0.95 0.40–0.53(Nb/Nb*)pm 0.60–0.79 0.32–0.67 0.92–1.15 0.28–0.42 0.21–0.55 0.31–0.65 0.23–0.40 0.02–0.51 0.38–0.48(Ti/Ti*)pm 0.86–0.92 0.59–0.82 0.84–0.94 0.59–0.74 0.89–0.98 0.80–1.59 0.67–0.86 0.42–0.82 0.03–0.07(Zr/Zra)pm 0.97–1.07 0.94–1.63 1.06–1.19 1.13–1.47 0.66–1.23 0.84–3.29 0.71–1.0 0.70–1.81 0.78–1.43

a Less altered ultramafic amphibolites from the Ivisaartoq greenstone belt (see Ordóñez-Calderón et al., 2008).


Quartzitic gneisses in the Ivisaartoq belt are silica rich (SiO2=72.0–81.6 wt.%). Relative to the biotite schists, they are enriched inZr (237–702 ppm) and REE (288–409 ppm), and depleted in Al2O3

(10.9–11.7 wt.%), MgO (0.5–1.6 wt.%), TiO2 (0.15–0.61 wt.%), andtransition metals (Ni, Co, and Sc b15 ppm) (Table 6). They have Zr/Tiand Nb/Y ratios similar to those of rhyolites and dacites (Fig. 5). Onchondrite and primitive mantle normalized diagrams (Fig. 9e–f),they display slightly fractionated REE patterns (La/Smcn=1.11–1.47;Gd/Ybcn=0.80–1.75). In addition, they possess pronounced negativeNb (Nb/Nb⁎=0.38–0.48), Eu (Eu/Eu⁎=0.40–0.53), and Ti (Ti/Ti⁎ b0.1)anomalies, and negative to positive Zr (Zr/Zr⁎=0.78–1.43) anomalies.

6. Discussion

6.1. Relationship between deformation and metamorphism

The juxtaposition of Eo- to Neoarchean (3850–2800 Ma) allochto-nous terranes of the Nuuk region occurred during several collisionalevents that are reminiscent of Phanerozoic-style continent–continentcollisional orogens (Friend et al. 1987, 1988; Nutman et al. 1989;Crowley, 2002; Nutman et al., 2004; Friend and Nutman, 2005;Nutman, 2006; Nutman and Friend, 2007). In the Ujarassuit green-stone belt, the effects of collisional tectonics are well indicated byhigh-grade mylonites (Fig. 3e–f), migmatitic TTG–gneisses andamphibolites (Fig. 2f–h), strong ductile tectonic attenuation andtransposition (Fig. 3a), recumbent folding (Fig. 3b–c), and completeobliteration of primary depositional features.

Relict D1 structures (Figs. 2g and 3a; Table 2) likely formed duringca. 2960Ma amphibolite facies metamorphism, which appears to havebeen related to the collision of the Isukasia and Kapisilik terranes(Friend and Nutman, 2005; Nutman and Friend, 2007). The orienta-tions of F3 isoclinal folds in the Ujarassuit greenstone belt areremarkably similar to those reported in all tectono-stratigraphicterranes to the southwest of the studied area (Friend and Nutman,1991). This suggests that D3, and the subsequent D4 deformation,occurred in the late Archean during or after the final amalgamation of

the different tectono-stratigraphic terranes of the Nuuk regionbetween 2650 and 2600 Ma (see Nutman and Friend, 2007). Incontrast, the presence of relict F1 rootless folds, transposed intoparallelism with D3 and D4 structures, indicates that the Ujarassuitbelt is likely formed by slivers of different metavolcanic andmetasedimentary units now imbricated and in close proximity.

6.2. Assessing element mobility in amphibolites

It is widely accepted that the concentration of HFSE (Nb, Ta, Th, Zr,Ti), REE (mainly Gd–Lu), and transition metals (Ni, V, Cr, Sc, Co) is notsignificantly changed during sea floor hydrothermal alteration andregional metamorphism (Hart et al., 1974; Condie et al., 1977; Luddenand Thompson, 1979; Ludden et al., 1982; Middelburg et al., 1988;Ague, 1994; Arndt, 1994; Staudigel et al., 1996; Alt, 1999; Polat andHofmann, 2003). However, some studies have shown that high-temperature alteration and high-grademetamorphism canmodify theconcentrations of these normally ‘immobile’ elements (Rubin et al.,1993; Van Baalen,1993; Galley et al., 2000; Jiang et al., 2005; Ordóñez-Calderón et al., 2008). In the Ujarassuit greenstone belt, theoccurrence of calc-silicate alteration, sulphide-bearing quartz-richlayers (Fig. 3g–h), and migmatites (Fig. 2f–h) suggest that the near-primary geochemical signatures of volcanic rocks could have beendisturbed.

Several studies have shown that partial melting of amphibolitesproduces andesitic to tonalitic melts in equilibrium with restiticamphibolites rich in garnet (e.g. Hartel and Pattison, 1996; Storkeyet al., 2005). For example, restitic amphibolites in the Harts RangeMeta-Igneous Complex, central Australia, display near-flat LREEpatterns and systematic enrichment of HREE (Lu up to 100 xchondrite) with increasing compatibility in chondrite-normalizeddiagrams (Storkey et al., 2005). These geochemical characteristicsresult from preferential retention of the heavier REE in garnet duringpartial melting. Group 1–4 amphibolites have low modal abundanceof garnet (0 to 2%), lack of significant enrichment of HREE (Fig. 7), andwere collected in outcrops without quartzo-feldspathic segregations.

Fig. 9. Chondrite- and primitive-mantle normalized diagrams for metasedimentary rocks. (a–b) Biotite schists in the western flank of the Ujarassuit greenstone belt. (c–f) Biotiteschists and quartzitic gneisses in the lower metasedimentary unit of the Ivisaartoq belt. Normalization values as in Fig. 7.


These characteristics indicate that Groups 1–4 amphibolites are notthe residues after partial melting.

Group 1 amphibolites (Fig. 7a–b) in the Ujarassuit greenstone beltare geochemically similar to the least altered pillow basalts and Group1 amphibolites in the Ivisaartoq greenstone belt (Polat et al., 2007;Ordóñez-Calderón et al., 2008). The presence of well preserved pillowlavas (Fig. 2a) and relict igneous clinopyroxene in the Ivisaartoq belthas allowed reliable identification of near-primary magmatic geo-chemical signatures (Polat et al., 2007, 2008). Therefore, the coherentnear-flat REE patterns and negative Nb–Ta anomalies in Group 1amphibolites (Fig. 7a–b) likely reflect the near-primary magmaticgeochemical signature.

Group 2 amphibolites display depleted LREE patterns (Fig. 7c–d)similar to those reported in ca. 3071Ma amphibolites (metabasalts) ofthe Qussuk greenstone belt in the Akia terrane (Garde, 2007), andGroup 3 amphibolites of the Ivisaartoq belt (Ordóñez-Calderón et al.,2008). There are two alternative explanations for the trace ele-ment patterns of Group 2 amphibolites. First, LREE were lost duringamphibolite facies metamorphism (e.g., Ordóñez-Calderón et al.,2008). Second, Group 2 amphibolites retain their magmatic geo-chemical signatures (cf. Garde, 2007). It is noteworthy that amphi-bolites with stronger LREE depletion (La/Ybcnb0.70) possess the mostpronounced negative Ti anomalies (Ti/Ti⁎b0.75) (Fig. 10). Theseanomalies are not related to fractionation of Fe–Ti oxides given the

positive correlation of Ti with Zr (Fig. 6). Mineralogical studies haverevealed that titanite and hornblende accounts for large amounts ofthe LREE, Ti, Th, Nb, and Ta in amphibolites (Mulrooney and Rivers,2005; Storkey et al., 2005). Breakdown of hornblende and titaniteduring high-grade metamorphism may have caused significant lossesof LREE, Nb, Ta, and Ti, resulting in LREE-depleted patterns and morepronounced negative Nb, Ta, and Ti anomalies (Fig. 7c–d). In theIvisaartoq belt, there is evidence for meter-scale mobility of LREE, Nb,and Ta, and to a minor extent Ti, during the prograde stage of regionalmetamorphism (Ordóñez-Calderón et al., 2008). Alternatively, thepronounced negative Ti anomalies could also indicate residualamphibole in the mantle source of the basaltic protoliths. The traceelement patterns of Group 2 amphibolites are comparable to those ofCretaceous LREE-depleted island arc tholeiites (IAT) in the Caribbeanisland arc, which were erupted during the early stages of arcdevelopment (cf. Viruete et al., 2006). Although we cannot rule out aprimary origin for the trace element characteristics of Group 2amphibolites, their geochemical similarity towell documented alteredamphibolites in the Ivisaartoq belt, and field evidence formetasomaticalteration (Fig. 3g–h) suggests that the trace element patterns of Group2 amphibolites resulted from postmagmatic element mobility.

Group 3 amphibolites (Fig. 7e–f) have trace element patterns verysimilar to those of average modern N-MORB (Hofmann, 1988). Theconsistent parallelism of trace element patterns, lack of pronounced

Fig. 10. Ti/Ti⁎ versus La/Ybcn diagram for Groups 1–3 amphibolites in the Ujarassuitgreenstone belt. Group 1 and 3 amphibolites from the Ivisaartoq belt are composition-ally similar, respectively, to Groups 1 and 2 in this study (see Ordóñez-Calderón et al.,2008). This diagram illustrates that amphibolites with stronger negative Ti anomaliesare more depleted in LREE (see Section 6.2).


anomalies of Ce and Eu, and the absence of calc-silicate alterationsindicate that they retain near-primary geochemical compositions.

Group 4 amphibolites do not display evidence for silicification, calc-silicate alteration, or anomalous enrichment ofmajor elements such asSiO2, CaO, Na2O, or K2O (Table 4; Fig. 6). Thus, their low abundance ofHFSE and REE is not the result of trace element dilution owing tometasomatism. In addition, several samples (498228, 498239, and498248) collected along a traverse of approximately 8 km (Fig. 1)display remarkably similar trace element patterns (Fig. 7g–h). There-fore,we suggest that Group4 amphibolites have retained their primarygeochemical signatures. Their low Ti and Zr contents, enriched LREEpatterns, sub-chondritic Gd/Ybcn ratios, negative Nb–Ta anomalies,and high Al2O3/TiO2 ratios (44 to 71) are consistent with a boninite-likegeochemical signature (cf. Fallon and Crawford 1991; Polat et al., 2002;Smithies et al., 2004; Manikyamba et al., 2005).

Plagioclase-rich amphibolites exhibit crossed LREE patterns thatare likely the result of LREE mobility (Fig. 8a–b). However, theirtransitional to calc-alkaline compositions (Zr/YN5.0) and overallenrichment of LREE and HFSE (mainly Th, Nb, Ta, and Ti) are con-sistent with basaltic andesite and andesite precursors (Fig. 5).

6.3. Origin of meta-ultramafic rocks

In the Ujarassuit greenstone belt, an actinolite–tremolite-richamphibolite (498210) displays similar mineralogical and geochemicalcharacteristics to those of variably altered ultramafic amphibolites inthe Ivisaartoq greenstone belt (Fig. 8c–d; Table 6) (cf. Fig. 9c–d inOrdóñez-Calderón et al., 2008). In the Ivisaartoq belt, some of thesemeta-ultramafic rocks preserve relict pillow structures, whichindicate a volcanic origin (Polat et al., 2007). These ultramaficamphibolites have been interpreted as metamorphosed island arcpicrites; given their high MgO (14.8–25.8 wt.%), Ni (469–1211 ppm),and Cr (1800–12000 ppm) contents, and their chondrite- andprimitive-mantle normalized patterns, which show moderate enrich-ment of LREE (La/Smcn=1.6–3.4), and pronounced negative Nb–Ta(Nb/Nb⁎=0.2–0.4) anomalies (Polat et al., 2007, 2008; Ordóñez-

Calderón et al., 2008). Accordingly, sample 498210 (MgO=19.7 wt.%)is interpreted as a metamorphosed picritic volcanic rock.

Picrites are olivine-rich high-MgO rocks with N12 wt.% MgO (LeBas, 2000). They are strongly porphyritic and may contain more than50 vol.% of olivine crystals (cf. Cameron,1985, Kamenetsky et al., 1995;Rohrbach et al., 2005). The high abundance of olivine is the result ofaccumulation of liquidus olivine, and xenocrystic olivine disaggre-gated from the upper mantle owing to high degrees of partial melting(cf. Boudier, 1991; Rohrbach et al., 2005). Geochemical studies ofolivine and associated melt inclusions in Mesozoic–Cenozoic picriteshave shown that the primary melts are ultramafic in composition,rather than basaltic, with 13 to 24 wt.% MgO (Kamenetsky et al., 1995;Rohrbach et al., 2005). Metabasalts and metapicrites in the Ivisaartoqgreenstone belt display different initial εNd values ruling out aconsanguineous origin through fractional crystallization (Polat et al.,2007, 2008). It is likely that actinolite–tremolite-rich amphibolites inthe Ujarassuit and Ivisaartoq greenstone belts represent metapicriteswith near-melt composition.

In contrast, serpentinites (Fig. 4c–d) consistently have highercontents of MgO (31.8–39.1 wt.%) and lower contents of CaO (0.49–2.85 wt.%), relative to the actinolite–tremolite-rich amphibolites(CaO=4.51–12.42 wt.%) (Table 6). The serpentinites have uniformMgO/SiO2 ratios (0.71–0.82; RSD=7%), which indicates that Mg and Siremained immobile during serpentinization (cf. Page, 1976; Iyer et al.,2008). Large variations in the CaO/SiO2 ratios (0.01–0.07; RSD=44%)suggest that Ca was significantly lost. The high MgO, Ni, and Crcontents of the serpentinite rocks (Tables 4 and 6) indicate that theyrepresent metamorphosed olivine-rich cumulates (cf. Findlay, 1969;Page, 1976; Wyllie, 1979; Müntener et al., 2001; Parlak et al., 2002;Yang, 2006). The concentrations of REE and HFSE in serpentinites aremore depleted, with some overlap, than those of the actinolite–tremolite-rich amphibolites (Fig. 8c–d). This low concentration oftrace elements is consistent with a cumulate composition. It is likelythat the large variations of LREE patterns are the result of serpentini-zation (Fig. 8c–d). However, HREE display coherent parallel patternson chondrite normalized diagrams, which is consistent with animmobile behavior during serpentinization. It is noteworthy that twosamples (498257 and 498262) with fresh olivine and low degrees ofserpentinization display similar trace element patterns (Fig. 8c–d;Table 4) to those of the least altered actinolite–tremolite-rich amphib-olites from the Ivisaartoq greenstone belt (Polat et al., 2007, Ordóñez-Calderón et al., 2008). These two samples display enriched LREE(La/Smcn=1.75–1.83), near flat HREE (Gd/Ybcn=0.91–1.32) patternsand pronounced negative Nb–Ta (Nb/Nb⁎=0.31–0.60) anomalies. Ac-cordingly, the serpentinites in the Ujarassuit greenstone belt are inter-preted to represent metamorphosed olivine-rich ultramafic cumulates,segregated from subduction-related picritic melts.

Given the high grade metamorphism and polyphase deformation,it is not clear if the serpentinite rocks represent fragments of layeredintrusions. In the Akia Terrane (ca. 3200–3000 Ma), northwest of theUjarassuit belt, olivine-rich cumulate rocks are part of layeredultramafic complexes (Garde, 1997). Alternatively, the protoliths ofthe serpentinites may well have been lava flows with large amounts ofentrained xenocrystic and cumulate olivine. For example, stronglyporphyritic picritic lavas in the Troodos ophiolite and Kamchatka, with40 to 70 vol.% olivine, show similar contents of MgO (33 to 36 wt.%) tothose of the serpentinites (cf. Cameron, 1985; Kamenetsky et al., 1995).

6.4. Biotite schists and quartzitic gneisses: sedimentary versusmetasomatic origin

In Archean high-grade terranes like the Nuuk region, recognition ofauthentic metasedimentary rocks, from metamorphosed metasomaticalteration zones, is not easy because polymetamorphismandmultistagedeformation have obliterated the primary sedimentary features (e.g.,Nutman et al., 1997; Fedo, 2000; Fedo andWhitehouse, 2002a,b; Friend

Fig. 11. Ternary weathering diagram (a) and selected trace element ratios (b–d) for biotite schists and quartzitic gneisses from the Ivisaartoq and Ujarassuit greenstone belts. Theternary diagram is plotted inmolecular proportions of Al2O3 (A)–CaO⁎+Na2O (CN)–K2O (K)where CaO⁎ represents the amount of CaO in the silicate fraction (Nesbit and Young,1982,1984; Fedo et al., 1995). The scale for the chemical index of alteration (CIA) is illustrated to the left (McLennan and Murray, 1999). Numbered stars represent the following reservoircompositions: 1=average oceanic island arc tholeiitic basalt (Kelemen et al., 2003); 2=average andesite (Kelemen et al., 2003); 3=averageN3.5Gaupper continental crust (Condie,1993); 4 = average Archean calc-alkaline granite (Kemp and Hawkesworth, 2003). In the ternary diagram: AS = Archean shale; arrows parallel to the A-CN side represent thepredictedweathering trend for intermediate to felsic protoliths (stars 2–4); arrow5 represents the trend of extremelyweathered rocks, and arrow6 the trend of sediments affected bypotassium metasomatism. Groups 1–3 amphibolites are those in this study and Group 1 amphibolites in the Ivisaartoq belt (Ordóñez-Calderón et al., 2008). Plagioclase-richamphibolites, with an andesitic geochemical composition, are from this study and those from the Qussuk greenstone belt (Garde, 2007).


et al., 2002; Bolhar et al., 2005; Cates andMojzsis, 2006; Manning et al.,2006; Friend et al., 2008). In the Ivisaartoq belt, evidence for siliciclasticor volcaniclastic–sedimentary origin is suggested by relict felsic cobblesset in a biotite-rich matrix (Fig. 2b–c). However, given the lack of wellpreserved sedimentary features, the biotite schists and quartziticgneisses could also represent the metamorphosed equivalents ofmetasomatized basalts, or intermediate to felsic volcanic rocks.

Although mass changes owing to hydrothermal alteration canconcentrate or dilute the composition of immobile elements (e.g., Al,Ti, Nb, Y, REE, Sc, and Ni), the inter-element ratios may remainrelatively constant carrying information on the composition of theprotolith (cf. Winchester and Floyd, 1977; Finlow-Bates and Stumpfl,1981; MacLean and Kranidiotis, 1987; MacLean, 1990). For example,Zr/Ti and Nb/Y ratios in biotite schists and quartzitic gneisses (Fig. 5)are consistent with andesitic to rhyolitic composition. Relative toGroups 1–4 amphibolites, biotite schists and quartzitic gneissesdisplay different covariations of TiO2, HREE, and Sc on variationdiagrams of Zr (Fig. 6) ruling out amphibolite protoliths (c.f. MacLeanand Barret, 1993; Ague, 1994; Roser and Nathan, 1997). Therefore, it isunlikely that these rocks represent metamorphosed alteration zonesdeveloped in basaltic rocks.

The chemical index of alteration (CIA) is a parameter widely used toquantify the degree of weathering of sediment sources (Nesbitt and

Young, 1984; Fedo et al., 1995; Nesbitt et al., 1996). Biotite schists andquartzitic gneisses possess CIA values (47 to 67) comparable or slightlyhigher than those of unweathered igneous rocks (38 to 51) but lowerthan average Archean shales (ca. 76) (Condie, 1993) (Fig. 11a; Table 5).The characteristic low CIA values and moderate Al2O3 content (12 to24wt.%) strongly suggest that quartzitic gneisses and biotite schistsmayrepresent metamorphosed volcanic flows or immature volcaniclastic–sedimentary rockswith scarce clayminerals in theirmodal composition.In addition, most samples plot across the predicted weathering trendsfor andesitic and granitic rocks in the A–CN–K diagram (Fig. 11a). Thistype of compositional trend in sedimentary rocks has been interpretedto indicate mixing of detrital sediments derived from various sources(McLennan et al., 1993, 2003).

Mixing of detrital sediments can also be monitored using inter-element ratios of transition metals (e.g., Cr, Co, and Sc) against HFSE(e.g., Th and Zr) and LREE (Condie and Wronkiewicz, 1990; Hofmann,2005). These elements are excellent tracers of source compositionbecause of their contrasting concentrations in mafic and felsic rocksand immobile behavior during alteration. Accordingly, biotite schistsand quartzitic gneisses plot along a linear array defined by basaltic andgranitic endmembers on the Co/Th versus La/Sc diagram (Fig.11b). Thisdiagram also shows significant compositional overlap between thebiotite schists and amphibolites with andesitic composition of the

Fig. 12. (a) Th/Yb versus Nb/Yb diagram showing the fields of modern MORB-OIB andvolcanic arc arrays (see Pearce and Peate, 1995 and Pearce, 2008). (b) Ce versus Ybdiagram with the compositional field for oceanic and continental arc basalts fromHawkesworth et al. (1993). The composition of least altered Group 1 amphibolites fromthe Ivisaartoq greenstone belt was plotted for intercomparisons (see Ordóñez-Calderónet al., 2008). Average composition of modern N-MORB and Archean (N3.5 Ga) uppercontinental crust (A-UCC) after Hofmann (1988) and Condie (1993), respectively.


Qussuk Peninsula (Garde, 2007). However, biotite schists and quartziticgneisses possess higher La/Sc ratios than andesitic rocks suggesting afelsic component in their provenance (Fig. 11b). The La/Sc ratios appearto indicate that the felsic component in quartzitic gneisses is moredifferentiated than the average early Archean (N3.5 Ga) upper con-tinental crust (Condie, 1993; Taylor and McLennan, 1995). This com-ponentmay be represented by highly fractionated calc-alkaline granitesand rhyolites. The presence of fractionated granitic sources is alsoimplied by increasing negative Eu-anomaly (cf. Taylor et al., 1986) withdecreasing Co/Th ratios (Fig. 11c).

On the Th/Sc versus Zr/Sc diagram (Fig. 11d), biotite schists alsodisplay a source-controlled compositional trend that reflects maficthrough felsic source rocks. In contrast, the quartzitic gneisses possesshigher Zr/Sc ratios (36 to 155) relative to the biotite schists (4.5 to 18)(Fig. 11d). High Zr/Sc ratios in modern sediments have been interpretedasevidence for zirconadditionowing to sedimentary sorting(McLennanet al.,1993, 2003).Quartzitic gneisses havehigh contents ofmodal zirconand large concentrations of Zr (up to 702 ppm) which may indicatezircon accumulation (Table 5). However, their enrichment in REE and Th(Fig. 9e–f), and pronounced negative Ti anomalies resemble thegeochemical characteristics of Archean high-silica rhyolites (SiO2-74 wt.%; Zr up to 680 ppm) reported in the Superior Province, Canada(Thurston and Fryer, 1983; Kerrich et al., 2008).

The evidence discussed above indicates that biotite schistsrepresent immature volcaniclastic–sedimentary rocks, most likelygraywackes, derived from poorly weathered intermediate to felsicsources and minor volcanic debris sourced from contiguous basalticrocks (Table 1). These immature sedimentary rocks generally occur atconvergent margins and normally represent first-cycle sedimentarydeposits (e.g., Cox et al., 1995). In contrast, major and trace elements(Figs. 5 and 11b–d) suggest that quartzitic gneisses represent compo-sitionally more mature volcaniclastic–sedimentary rocks, quartz-richarkoses, sourced from felsic rocks. In the absence of zircon age data, thehigh Zr/Sc ratios (Fig.11d) have two possible explanations. First, quartz-itic gneisses may represent reworked volcaniclastic–sedimentary rocksderived from Mesoarchean highly-fractionated rhyolites. Strong acidichydrothermal alteration of felsic source rocks could have producedresidual detritus enriched in SiO2 and Zr (e.g., Sugitani et al., 2006).Second, high SiO2 and Zr contents may reflect sedimentary reworkingand recycling of older continental rocks as has been indicated for theenrichment of Zr in modern turbidites (McLennan et al., 1990). Thiswould imply that the Ivisaartoq greenstone belt likely formed close tocontinental crust.

6.5. Petrogenesis, mantle processes, and geodynamic setting

Groups 1–4 amphibolites do not exhibit compositional variationsindicating mixing trends with average Archean upper continentalcrust (e.g., Fig. 12a–b). They possess low Ce/Yb ratios (Fig. 12b)reminiscent of primitive lavas erupted in intra-oceanic arcs such asTonga-Kermadec and South Sandwich Islands (Hawkesworth et al.,1993; Hawkins, 2003; Pearce, 2003). The fault-bounded contactbetween TTG–gneisses and amphibolites (Fig. 2d–e), and amphibolitexenoliths within some TTG–gneisses (Fig. 2g) suggest that volcanicrocks were not erupted over an underlying continental basement.These field and geochemical characteristics indicate that volcanicrocks likely formed in an intra-oceanic setting.

Phanerozoic volcanic rocks erupted at active continental marginsand intra-oceanic island arcs show characteristic negative Nb–Taanomalies in primitive mantle normalized diagrams (Kelemen et al.,2003). The distinctive trace element characteristics of primitive arclavas reflect partial melting of the mantle wedge, which is variablymetasomatized as the oceanic lithosphere is subducted and ex-periences dehydration and partial melting (Pearce and Peate, 1995;Becker et al., 2000; Schmidt and Poli, 2003). Accordingly, the negativeNb–Ta anomalies in arcmagmas result from inefficient transfer of HFSE

elements, relative to Th and LREE, from slab-derived fluids into themantle wedge. Group 1 amphibolites (Fig. 7a–d) display negative Nb–Ta anomalies and consistently plot in the field of volcanic arc lavas onthe Th/Yb versus Nb/Yb diagram (Fig. 12a) (Pearce and Peate, 1995;Pearce, 2008). They are interpreted as island arc tholeiites (IAT).

Given the low solubility of Th in slab-derived aqueous fluids, and itslow concentration in the depleted mantle, the enrichment of Th inintra-oceanic arc lavas (Fig. 12a) appears to require refertilization of themantle wedge with hydrous melts derived from subducted siliciclasticsediments and oceanic crust (Plank and Langmuir, 1993; Hawkesworthet al., 1997; Becker et al., 2000; Kelemen et al., 2003; Dilek et al., 2007;Klimm et al., 2008). A sedimentary component fluxed into the mantlesource of the Ivisaartoq belt pillow basalts appears to be indicated byvariable initialεNd values (+0.30 to+3.10),whichhavebeen interpretedas evidence for recycling of older continental crust via subduction


(Polat et al., 2008). In the Archean, melting of the slab, and crustal inputinto the mantle wedge via subduction, may have been more commonthan in the modern Earth owing to higher mantle temperatures andhotter lithospheric plates (Martin, 1999; Schmidt and Poli, 2003; Dilekand Polat, 2008; Pearce, 2008).

High-Mg Group 1–3 amphibolites (MgON8.0 wt.%) show muchhigher Fe2O3 (11.1–13.4 wt.%) than modern N-MORB (9.8 to 11.6 wt.%)and primitive IAT (8.9–10.6 wt.%) (Hofmann, 1988; Kelemen et al.,2003). They are also depleted in HREE and HFSE (mainly Ti, Zr, Nb, andTa) compared to modern N-MORB (Fig. 7a–f). These geochemicalcharacteristics indicate that the basaltic protoliths of Groups 1–3amphibolites resulted from larger degrees of partial melting relative tomodern tholeiites, and that the Mesoarchean mantle beneath theIvisaartoq and Ujarassuit greenstone belts (sensu lato) was moredepleted than the source of modern N-MORB (see Polat et al., 2007;2008).

A strongly depletedMesoarchean sub-arc mantle implies an earlierhistory of partial melting before arc initiation, possibly associatedwithArchean sea floor spreading (cf. Ohta et al., 1996; Furnes et al., 2007).It is noteworthy that Group 3 amphibolites do not exhibit the negativeHFSE anomalies that characterize subduction-related magmas(Fig. 7e–f). Instead, their trace element patterns resemble those ofArchean (3.1 to 3.3 Ga) mid-ocean-ridge basalts (A-MORB) from thePilbara Craton (Ohta et al., 1996) and modern N-MORB (Hofmann,1988). Accordingly, Group 3 amphibolites may represent relict pre-arcA-MORB-like oceanic crust, trapped during the initiation of subduc-tion, formed at a mid-ocean ridge or back-arc spreading system. Or

Fig. 13. Schematic block diagram (not to scale), across a hypothetical intra-oceanic supra-sgreenstone belt. The polarity of subduction is arbitrary. The protoliths of metavolcanic rockschists represent volcaniclastic–sedimentary rocks with a mixed provenance, which includederived from mafic to ultramafic volcanic rocks in the forearc or back-arc.

alternatively, they may have formed during an episode of arc rifting(cf. Dilek and Flower, 2003).

In Group 4 amphibolites, high MgO (10.8–13.0 wt.%) and Cr (99–765ppm), and lowTiO2 (0.26–0.33wt.%), Zr (20.3–28.6ppm), andHREE(HREE1.9 to3.2×primitivemantle) contents are consistentwithmodelsfor boninite generation by high degrees of partial melting of a refractorymantle source (Sun and Nesbitt, 1978; Bloomer and Hawkins, 1987;Crawford et al., 1989; Pearce et al., 1992; Kim and Jacobi, 2002; Dileket al., 2007). In addition, the LREE enriched patterns (La/Smcn=1.8 to2.4) andnegativeNb–Ta anomalies (Fig. 7g–h) suggest that thedepletedmantlewedgewasmetasomatized by fluids and possibly hydrousmeltsfluxed from the subducting slab, prior to or during partial melting (cf.Taylor and Nesbitt, 1988; Bédard, 1999). Boninites in modern arcs mayform in the forearc, owing to forearc extension during the early stages ofsubduction (Stern and Bloomer, 1992; Kim and Jacobi, 2002; Reaganet al., 2008). They may also be erupted in the back-arc, as the back-arcspreading center propagates into the volcanic arc and interact with slabderived fluids (Deschamps and Lallemand, 2003).

In the Ujarassuit and Ivisaartoq belts, the absence of voluminousintermediate to felsic rocks is consistent with an immature stage of arcdevelopment. Plagioclase-rich amphibolites (Table 1) with basalticandesites and andesite compositions are rare. Nonetheless, theydisplay negative Nb–Ta anomalies, enriched LREE, and fractionatedHREE (Gd/Ybcn=2.6–3.1) patterns (Fig. 8a–b) consistent with asubduction zone geochemical signature and deep melting (≥80 km)with residual garnet in the source (cf. Johnson, 1994; Hirschmann andStolper, 1996; Van Westrenen et al., 2001; Kelemen et al., 2003).

ubduction zone setting, illustrating the proposed geodynamic origin of the Ujarassuits are interpreted to have been erupted either in the forearc or back-arc region. Biotites detritus derived from the erosion of felsic to intermediate arc rocks, and local detritus


Despite their LREE enrichment, plagioclase-rich amphibolites possesscomparable TiO2 and slightly lower HREE contents relative to Groups1–2 amphibolites (Table 4; Figs. 7 and 8). Therefore, they are notrelated by fractional crystallization. Instead, the high Mg-numbers(48–62) and high contents of Th, Nb, LREE, and transition metalssuggest that intermediate rocks likely formed through interactionbetween melts derived from hydrated basaltic slab and the sub-arcmantle (e.g., Defant and Drummond, 1990; Martin, 1999; Smithies,2000; Condie, 2005b; Martin et al., 2005).

High contents of MgO (N19 wt.%) and transition metals(Cr=2377–6312 ppm, Ni=658–2200 ppm), and low concentrationsof REE (HREE 0.6 to 2.0 x primitive mantle) and HFSE (b0.4 wt.% TiO2)in meta-ultramafic rocks are consistent with a picritic composition(Fig. 8c–d; Table 1). Mesozoic–Cenozoic picrites have formed invarious geodynamic settings including, but not restricted to, mid-ocean ridges, oceanic plateaus, and island arcs (Kerr et al., 1996; Perfitet al., 1996; Thompson et al., 2001). The least altered meta-ultramaficrocks (498262 and 498257) possess the characteristic LREE-enrichedpatterns and negative Nb–Ta anomalies of island arc picrites in thelesser Antilles, Solomon, New Hebrides, and Kamchatka (e.g., Eggins,1993; Kamenetsky et al., 1995; Woodland et al., 2002; Schuth et al.,2004). The least altered picrites in the Ivisaartoq belt display largepositive initial εNd values (+4.21 to+4.97) consistent with long-termdepletion of the Ivisaartoq sub-arc mantle (Polat et al., 2007, 2008). Itis worth noting that modern subduction-related picrites appear to berestricted to intra-oceanic settings (Rohrbach et al., 2005).

The lithogeochemical association of IAT, transitional to calc-alkalinebasaltic andesites andandesites, arc-likepicrites, andboninite-like rocksis consistent with a supra-subduction zone tectonic setting for theUjarassuit and Ivisaartoq greenstone belts (Table 1; Figs. 7 and 8). Giventhat picrites and boninites are the products of large degrees of partialmelting, their occurrence in modern island arcs is related to strongthermal anomalies in the mantle wedge. Subduction of young, hotoceanic lithosphere, spreading-ridge subduction, subduction initiationacross fracture zones, and supra-subduction zone extension appear toexplain the unusual high-temperatures in the sub-arcmantle (e.g., Sternand Bloomer, 1992; Kim and Jacobi, 2002; Hall et al., 2003; Deschampsand Lallemand, 2003; Schuth et al., 2004; Ishizuka et al., 2006; Dileket al., 2007). Accordingly, we postulate that the volcanic rocks in theUjarassuit and Ivisaartoq greenstone belts likely representMesoarcheanoceanic crust formedeither in the forearc or back-arc region (Fig.13) (seePolat et al., 2007, 2008).

6.6. Collisional orogenesis of the Nuuk region and implications forArchean ophiolites

The Phanerozoic-like continental collisional tectonics proposed forthe Nuuk region (Friend and Nutman, 2005; Nutman and Friend,2007) predicts the existence of convergent margins and marginalbasins in the Archean record. Intra-oceanic terranes may have beentrapped into colliding continents during the closure of Archean oceanbasins in a similar fashion to Tethyan ophiolites in the Mediterraneanregion (Şengör, 1990; Dewey, 2003; Flower, 2003; Dilek and Flower,2003; Dilek et al., 2007).

Supra-subduction zone oceanic crust displays various petrologicalcharacteristics that record complex events during its formationincluding progression from mid-ocean ridge, subduction initiation,intra-arc volcanism, slab rollback, arc rifting, and back-arc basinopening (Stern and Bloomer, 1992; Shervais, 2001; Dilek and Flower,2003; Flower, 2003; Dilek et al., 2007). The Ujarassuit greenstone beltis composed dominantly of metamorphosed IAT which are spatiallyand temporally associated with N-MORB-like basaltic rocks, boninites,picrites, andesites, and volcaniclastic–sedimentary rocks (Table 1),implying that the belt likely contains upper-most crustal componentsof Mesoarchean forearc or backarc oceanic crust. A forearc tectonicsetting was proposed for volcanic rocks in the Ivisaartoq belt (Polat

et al., 2007, 2008), and the discovery of boninites in this studysubstantiates this interpretation. However, if the high contents of Zr inquartzitic gneisses from the Ivisaartoq belt resulted from recycling ofolder Archean felsic crust (Fig. 11d), the Ivisaartoq belt may haveformed close to a continental block.

Immature volcaniclastic–sedimentary rocks (biotite schists) in theIvisaartoq and Ujarassuit greenstone belts were derived mostly fromnearby felsic to intermediate intra-arc volcanic complexes and depos-ited onto adjacent basaltic oceanic crust (Fig. 13). These Mesoarcheanvolcanic arc complexes, however, appear to be missing from the geo-logical record in the study area. Nevertheless, uppermost crustalcomponents from a contemporaneous 3071 Ma island arc complex arewell preserved at Qussuk and Bjørneøen in the Akia terrane (Garde,2007). This relict volcanic arc complex is composed of metamorphosedandesitic volcanic rocks erupted on top of more primitive basaltic crustwith geochemical characteristics similar to those of Group 2 amphibo-lites in theUjarassuit greenstonebelt. The exposure of deep crustal rocksin the Ujarassuit greenstone belt suggests that the uppermost crustalrocks have been removed by erosion. This may well explain the absenceof intra-arc complexeswhichmayhavebeen located at shallower crustallevels (cf. Garde, 2007).

Comparable geochronological, lithological, and geochemical char-acteristics of metavolcanic and metavolcaniclastic–sedimentary rocksin the Ujarassuit and Ivisaartoq belts suggest that they likely representslivers of volcanic suites formed along the same Mesoarcheanconvergent margin. Therefore, the Ivisaartoq and Ujarassuit green-stone belts are interpreted to represent dismembered fragments ofMesoarchean supra-subduction zone oceanic crust. Accordingly, theNuuk region appears to include scattered fragments of incompleteMesoarchean ophiolites formed at different paleogeographic settings(forearc, intra-arc, and back-arc) along and across a supra-subductionzone.

7. Conclusions

The following conclusions are drawnbasedon thefield characteristicsand geochemical signatures of metavolcanic and metavolcaniclastic–sedimentary rocks from the Mesoarchean Ivisaartoq and Ujarassuitgreenstone belts:

1. These belts are dominated by metamorphosed IAT (Table 1).Basaltic andesites, andesites, picrites, boninites, and volcaniclastic–sedimentary rocks are a minor component. These lithologicaland geochemical characteristics suggest an intra-oceanic supra-subduction zone origin for the Ujarassuit and Ivisaartoq greenstonebelts (Figs. 7–9; Table 1). In addition, the presence of boninite-likerocks spatially associated with picrites and picritic cumulatesindicates that the Ujarassuit greenstone belt likely representsobducted fragments of Mesoarchean forearc or back-arc oceaniccrust.

2. In the Ujarassuit greenstone belt, recognition of the stratigraphicrelationships between subduction-related rocks including IAT,boninites, and picrites, and rare N-MORB-like tholeiites (Group 3amphibolites) are obscured by late Archean polyphase deformation(Figs. 7 and 8; Table 1). Accordingly, the protolith of Group 3amphibolites may well represent pre-arc oceanic crust formed atmid-ocean ridges or back-arc spreading centers (e.g., Leat andLarter, 2003); or alternatively it may have formed during an epi-sode of arc rifting.

3. The following geochemical evidence indicates that the Mesoarch-ean mantle beneath the Ivisaartoq–Ujarassuit greenstone belts washeterogeneous, more depleted in trace elements than moderndepleted upper mantle, and variably metasomatized by slab-derived fluids: (1) Groups 1–4 amphibolites (Figs. 7 and 8) havelower abundance of REE and HFSE (mainly Ti, Zr, Nb, and Ta) thanmodern average N-MORB (Hofmann, 1988); (2) contrasting


geochemical patterns of mafic and ultramafic rocks; (3) low-Tihigh-Mg rocks (boninites and picrites) indicative of refractorymantle sources; and (4) primitive mantle-normalized diagramswith negative Nb and Ta anomalies, flat LREE patterns, and enrich-ment of Th relative to HFSE and HREE.

4. Biotite schists display major and trace element characteristicsresembling immature volcaniclastic–sedimentary rocks (gray-wackes) derived from mixtures of poorly weathered (CIA=47to 67) andesitic to rhyolitic sources (Fig. 11a–d). In addition, highconcentrations of transition metals (Ni up to 328 ppm) suggestthat sedimentary rocks also contain volcanic debris derived fromlocal ultramafic to mafic rocks. Felsic to intermediate detrituswas likely sourced from distal volcanic arc edifices (Fig. 13), giventhat supracrustal belts are dominated by mafic rocks. Voluminousfelsic to intermediate volcanic sequences are not represented inthe area, and therefore appear to haven been eroded away. Thiscould be related to the present level of erosion in the Nuuk regionwhich exposes mainly middle to lower crustal rocks (see Garde,2007).

5. Quartzitic gneisses in the Ivisaartoq greenstone belt have highcontents of SiO2, Zr, and REE, high Zr/Sc ratios, and low con-centrations of transition metals indicating a felsic provenance(Figs. 9e–f and 11). These geochemical characteristics may havebeen inherited from highly-fractionated Mesoarchean rhyolites; oralternatively, recycling of older continental crust may have resultedin significant zircon addition (cf. McLennan et al., 1990, 2003).Given the oceanic origin of volcanic rocks, the later would implythat the Ivisaartoq volcanic rocks were erupted in the proximity ofolder Archean continental crust (cf. Polat et al., 2007, 2008).

6. The new field and geochemical data presented in this contributionand those of previous studies (Garde, 2007; Polat et al., 2007, 2008)have revealed that the Nuuk region comprises several dismemberedfragments of Mesoarchean arc–backarc–forearc oceanic crust. Theseinclude intra-arc volcanic complexes in Qussuk and Bjørneøen(Garde, 2007), and forearc or backarc complexes in the Ujarassuitand Ivisaartoq greenstone belts. This is consistent with theallochtonous terrane model whereby the closure of Archean oceanbasins in the last stage of aWilson cyclewould have trapped Archeanoceanic crust into the colliding continents (FriendandNutman, 2005;Nutman and Friend, 2007). Future studies may reveal scatteredvolcanic complexes formed at different paleogeographic locations(e.g., back-arc basins) alongandacross a regionalMesoarchean supra-subduction zone.

Acknowledgements

We thank Z. Yang, and J.C. Barrette for their help during the ICP-MSanalyses. This is a contribution of PREA and NSERC grant 250926 toA. Polat, and NSERC grant 83117 to B. Fryer. Field work was funded bythe Bureau of Minerals and Petroleum in Nuuk and the GeologicalSurvey of Denmark and Greenland (GEUS). A. Garde and A. Nutmanare thanked for constructive reviews which have resulted in signif-icant improvement to this paper.

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